The traditional (i.e., "old") method is to spread out aviation charts on a large table, and use a plotter to hand-craft a flight plan on paper. Victor airways (the VOR airway network) are depicted both on VFR sectionals and on IFR low & high altitude enroute charts. The pilot/navigator will mark his or her origin and destination, and eyeball the available airways to come up with a suitable route, much like a driver would use a national atlas to find what interstates/highways to take for a long road trip. These maps depict all the information needed about distances, courses, minimum altitudes and airspace classifications.
Planning by hand may also involve the use of the Airport/Facility Directory (A/FD), which has information about airports, and the Terminal Procedures Publication which shows what instrument approaches are available at each airport and any necessary IFR departure procedures, takeoff weather minimums, etc.
Nowadays, people will more often use computer planning tools like AOPA Flight Planner, fltplan.com, etc. There are a large number of sites and software programs for this, some free, some requiring paid subscription / purchase. Most recently, mobile apps for smartphones and tablets have emerged with the same capabilities.
The best such sites/software will also provide the necessary information about terrain, takeoff minimums, departure procedures and so on, and even give you current NOTAMs, weather forecasts, etc. Many also have aircraft performance databases (or allow you to create one yourself for your airplane) and can do performance calculations for you. Some will even find the cheapest places along your route to purchase fuel, and also give you information about local transportation and accomodations.
Note, though, that neither the FAA nor any other civil aviation authority authorizes the pilot to rely on software resources. The pilot is still always responsible for verifying that he or she has complete, accurate information from official sources. In other words, if you're using software to plan your flight, and you violate restricted airspace that your flight planning software didn't warn you about, you'll be held responsible for not verifying the information and catching the error. So be careful what you use and rely on!