There are actually a couple of different towing mechanisms out there. You've got your traditional banner pick, with the ropes and the hooks and all that, but there's also a new type of billboard mechanism that allows a tow aircraft to take off with the billboard already attached; it unfurls as the aircraft gains altitude, and can eliminate a potential inefficiency in the process (some days are worse for banner picks than others due to high winds, turbulence, etc.), but it limits the tow aircraft to some pretty small billboards and letter banners are pretty much a no-go.
Talking about the traditional method of banner towing, there are a couple of things to cover.
- The banner/billboard setup on the ground - they will be attached to a tow rope (usually about 200 feet long) with a loop at the end. This loop is strung out between two poles a small distance apart. The goal of a banner pick is to have the hook pass between the poles, thru the loop; do that and you will be able to pick the banner off the ground. The banner or billboard will be laid down upwind of the poles (with the tow rope fully extended), so that the pick takes place into the wind; the lower ground speed will help with timing and allow the banner or billboard to come off the ground more smoothly. A small windsock is a common addition next to the poles to make it easier for the pilot to spot them and to give some local wind information. The banners and billboards may be set up parallel to a runway (usually the active runway), or there may be a designated spot elsewhere on the field.
- The hook itself is attached to aircraft with a tow hitch and it is possible to have multiple hitches on the same aircraft (highest number i ever saw was 4 on a Piper Pawnee); they get used sequentially i.e. drop the hook, pick up the banner, fly the banner, drop the banner, drop the next hook, pick up the next banner, fly said banner, and so on until you run out of hooks. Most of the time, hooks that are not in use will be tied down somewhere accessible to the pilot who will drop them manually. That Pawnee I mentioned earlier, however, had the hooks attached to hitches on the aircraft's belly, from where they would be released with a switch, so there's more than one way to skin a cat. The hooks are normally attached prior to engine start and the pilot needs to make sure there is no slack in the line, to avoid it becoming entangled in rudder or elevator cables (more on that later), or other assorted bits and pieces of equipment.
- Banner tow procedures
- Take off is normal, but depending on your hook and cockpit setup, you may have to hold the hook in your throttle hand as you're taking off (this was the case with the Husky I towed in). Be careful not to hit anything with it.
- After climbing through about 50 feet AGL, side-step the runway, and drop the hook. Procedure varies depending on your setup, but we used to extend the arm holding the hook as far out the window as we could get, give a gentle tap on the rudder to kick the tail out of the way and toss the hook outwards and away from the airplane. Kicking the rudder was important due to the way the rudder control cables ran in the Husky; we had one pilot who didn't and nearly wrecked when the rope got fouled around the cables, forcing a rudder hard-over once there was tension in the line after the pick - fortunately he was a big guy and the runway was sufficiently long that he was able to overpower the rudder and make a safe landing. Tossing the hook outwards is important lest it get caught up in the propwash and bang against the skin of the aircraft. Again, this happened (wasn't me though).
- Coming around for the banner pick - depending on where the banners are set up, you may end up flying almost a regular traffic pattern, but it usually happens at much lower altitudes (5-600 feet) and the banner tow waiver may require that you avoid flying over populated areas to do so, so you may end up flying, say a right-hand pattern instead of your regular left. Either way you end up doing it, you want to end up downwind of the poles, similarly, if you will, to coming in for landing.
- The actual banner pick - this is where you'll see a lot of variation. Some outfits will come in flat, just above the poles and pull up once they pass them (unusual, but some do it that way), others will come in high and dive towards the poles, and pull up once past them (it's very pretty and dramatic, but not a lot of room for error IMO). We used to be a bit more sedate about it, and came in as if for a landing, with the goal of touching down at the top of the banner (i.e. the end nearest to you). Obviously, you have to correct for crosswind, which you can do by shifting your line left or right depending on wind direction; the goal is to place the hook, which is trailing behind you and getting blown about by the wind, between the poles. Either way you do it, once you pass the poles, you pitch up immediately to at least 45 degrees or so, and apply full throttle. You zoom climb to about 200 feet AGL and then reduce the pitch angle to be able to maintain a slow-speed climb. Do this right and the banner or billboard will come smoothly off the ground; you may feel a slight tug as tension is applied through the towline if you haven't been smooth enough and ended up yanking the banner off the ground. Continue to climb to whichever altitude you've planned for and, when stabilized in the climb, try to take a look behind you (if you can) to make sure you've picked successfully and that the banner didn't get damaged or something. If you can't look behind you, you can check during a turn, or have your ground crew radio/text and let you know.
- Fly the banner - you want to fly slow enough to avoid stressing the engine, airframe, tow rope and banner, but not quite so slow that the engine can't get enough airflow to cool down (remember, on the backside of the power curve, power required increases the slower you try to fly). Watch your altitude and distance from people and objects (the powers that be are quite serious about it), look out for towers (especially those that come with guide cables and the like), high rise buildings, and other traffic, and try not to get too bored flying in circles for hours.
- Drop the banner - when you come back in to drop the banner, the goal is to drop it low enough to the ground that the wind won't have time to blow it away onto active runways (or into the trees - I had to help pull somebody else's banner out of the trees once, not fun). Ideally, you might want to drop it as close as you can to where the ground crew has their bags and everything setup to spare them having to haul heavy banners and billboards over long distances. You release the hook using a lever bolted onto the floor of the aircraft and then apply power, climb out, and come around for another pick or a landing, whatever the case may be.
Hope this helps, let me know if you'd like to know more about any of this.