Imagine laying your track on a piece of plywood, just how you want it to go. You then cut out a 4" strip along the route of your track. Then you take that piece of plywood and put it above your frame and raise it to the correct height.
Cookie cutter is a simple method of creating inclines on the standard 4x8 sheet of plywood. You simple take a jigsaw and make cuts on either side of where the inclined track will go. You can then use risers to lift this cutout section gradually to the level you need it to reach. The bottom end of the incline remains attached to the main section of plywood.
I think the cookie-cutter question has been covered. The other part of your question: “‘How do you determine what railroad do you choose and where do you get pictures after you decide what line to go with’” is a whole other matter.
SOme people, myself included “freelance” our railroad. We determine what name and environment our railroad operates with. Alan B. for example models “Between Arock & Hardspot”.Arock is one town, and Hardspot is at the other end of the line. A past mentor of model railroaders Mr. Allen modeled the “Gorre & Dephetid” (the gore-y and defeated). If you read my signature you will see my choice.
Others will pick a particular railroad that they are somewhat familiar with. Maybe it was in their childhood nearby. Maybe they like what the line does (haul coal, ore or maybe trees/lumber). Some will narrow down a railroad to a particular sub or portion serving the railroad of their choice. They will only model that which is particular to it. They will do diligent research on the railroad and aim to portray that railroad, and that era down to the cigarette butts that may have been on the ground then.
That choice is up to you. You can do online research, or if some element of the railroad is still operating today, maybe research their archives.
Some will pick the steam era to model because they love the steamers. (I do). Others will pick modern era diesels. Many will