I can't tell you what specifically to include on your navlog (that's a judgement call you make as a pilot - you determine what has to go on the paper to satisfy yourself that you can complete the flight), but I can give you a crash course in my theory of navigation, and a practical example of how I decide what to put on my navlogs.
What's the point of the Navlog?
At its core a navlog is a list of waypoints such that you could find your way from Point A to Point B via the listed route by using some navigational system.
Ideally the navlog should be usable with at least two independent navigational systems in case one breaks, and for a VFR flight one of those navigational systems should always be "Looking Out The Darn Window!" (the window rarely breaks).
For purposes of this discussion I'm going to focus on waypoint selection and a few other housekeeping tasks. I'm ignoring stuff like calculating headings because while that stuff is important it's also just mechanical math: The waypoint selection requires you to exercise some judgment as a pilot.
A Practical Example: KGON to KMGJ and back.
This is a nice 100 mile flight to use as an example. Let's plot the outbound trip using pilotage and dead reckoning, and the return trip using the GPS. (This is going to get photo-heavy, so I'm burying them in links.)
The first thing you do is draw the straight line between the two airports on the chart. Then follow the line from Point A to Point B and mark off convenient landmarks. I chose Chester Airport, the New Haven railroad/highway/river, abeam the Waterbury-Oxford Airport, the lake & prison north of Danbury (also abeam the Danbury airport), and the Hudson River (you can't miss it!).
Pilotage Tip: Airports (at least the larger ones) make great landmarks. They're also convenient landing places if you have a problem, so including them as waypoints is a double win.
Almost all of these legs can be completed by pilotage (you can always see the next waypoint/landmark), but one of the legs (Danbury to the Hudson) is about 24 nautical miles and on a hazy summer day you might not be able to see that far. You should therefore be prepared to fly at least this leg by dead reckoning (point the nose in the right general direction based on the heading you calculated, and fly until you see a big river).
The navlog that Skyvector generates for this is somewhat unfriendly-looking, with lots of latitude/longitude fixes in it, and automatically generated ones from Foreflight and similar tools will look about the same. When you write up your navlog you can give each waypoint a descriptive name like I did above so you can remember what you're looking for in case your sectional blows out the window mid-flight.
So that takes care of the outbound trip with visual waypoints - what about the return trip using the GPS?
GPS systems have convenient databases of airports, intersections, and navaids, so we draw the same straight line, then look at the chart and find the waypoints between our two airports that the GPS will know about. The result looks like this and produces a navlog with nice waypoint names.
GPS Tip: I avoided using Lat/Long fixes in the GPS plan in favor of fixes that you can type in on any GPS, so this is fast and easy to enter on a Garmin 430/530 for example. Newer GPS systems like the touchscreen Garmins let you do "rubber-band flight planning" where you can drag your route of flight around and easily create waypoints wherever you want - you could even set up the same track I did for the outbound flight if you want.
Since you're flying under Visual Flight Rules you're going to want to note down Visual references to go with this navlog (because again if your instructor is anything like mine was your GPS is going to suddenly fail on the return trip while you're merrily following the magenta line).
For this return trip the visual references would be Stewart Airport (and the Hudson River beyond it, even though it doesn't have a dot on this plan), the city of Brewster (which doesn't have a dot, but is identifiable as a city that has power lines, railroad tracks, and a highway immediately east of it), the Danbury airport (KDXR), abeam the Bridgeport airport (at TRUDE), and from there on you can cheat and use the shoreline (even if the entire electrical system died you can still find your way back to Groton by following the shore until you recognize "home").
What about that other stuff? (Airspace, ToC/ToD, ETE, fuel burn, etc.)
There are a few important things you don't get by picking out the waypoints the way I did it above. The most obvious one is Airspace -- you'll notice the course I've plotted cuts right through the Stewart (KSWF) Class D airspace, and Orange County is really close to the Class D ring. Unless you're in a helicopter there is no way you could fly above the Stewart airspace and then descend into Orange County along the plotted route without exceeding VNE, so you're going to have to plan on talking to Stewart's tower and requesting a transition. Note the tower and ATIS frequencies and pick a spot along your course to call the tower up to request the transition. (Same goes for the return trip - we plan to cut through right over the airport so we need to call them and ask.)
Similarly you want to note the tower frequency of any airport you're passing so you can monitor it, and so you can call them up if something goes wrong. This information is on the chart, but printing it in big letters on your navlog makes it easier to find.
Something else most flight instructors want to see you including are Top of Climb and Top of Descent (along with a calculation of fuel used in the climb or descent) - you get this the day of the flight by determining what altitude you want to climb to (based on the wind data you got in your pre-flight briefing) and referencing the aircraft's performance charts. You'll add a little mark to the course line on your chart for TOC and TOD, and I usually put it in a "notes" area on my navlog (TOC xx minutes elapsed and if there's a convenient visual reference I note what it is).
General Tip: I try to include some waypoint that will be at or near top-of-climb/top-of-descent on my flight plans - if you have a reasonable idea of how your plane performs you can usually pick something within a few miles for most reasonable wind conditions. Doing that makes it easier because I can use the time-fuel-distance charts in the POH to fill in the estimated time enroute to that waypoint (or waypoints if it's a long climb) and the fuel burn: One less thing to calculate.
I did not go out of my way to do this on the charts above.
You should also be calculating an estimated time enroute between each waypoint based on your calculated groundspeed (again you'll work this out before you depart based on forecast winds - you'll see that Skyvector computed it for us based on a 100-knot airspeed and the wind data available as I was typing this).
This seems a litle crazy for 10 to 30 mile legs like this (about 6 to 20 minutes enroute between each waypoint, but if we go back to the idea of using this navlog over a wide-open desert we may have an hour or more between waypoints, and cross-checking estimated time versus actual time lets you know if the winds are significantly different from the forecast values.
The estimated time enroute in turn gives you your fuel burn for each leg: You know your engine burns a certain number of gallons per hour, and you know how many hours (or fractions of an hour) each leg will take - multiply to get your fuel burn.
Don't Run Out Of Gas Tip: Plan conservatively: Round fuel consumption numbers up to the next half-gallon or gallon.
Finally every navlog has at least one other key element: The heading you should be flying: True Course ± Magnetic Variation ± Wind Correction Angle ± Compass Deviation = Magnetic Heading - What you steer on the compass. Skyvector calculated this for us (with the exception of the Compass Deviation which it can't know) but if you're doing the flight plan manually you need to do this math yourself.
Again you would do this right before your flight, with the most current wind data.
