Once you have decided your track plan and have built your model layout's structure (sub roadbed) it's time to lay some roadbed. Do not lay your track right down on the wood. This is a big no no. I know you want to run some trains but this hobby requires some patients. It doesn't matter what scale you are modeling in you need roadbed below your rails.
Roadbed simulates ballast that every real railroad track has in the real world. It elevates the track just a bit from the surrounding area just like in real life. It also provides a bit of sound proofing during operation. This soundproofing can go a long way especially if you are modeling in O scale or your typical Lionel set up.
The cork is easily shaped & cut and conforms to your model train layout's curves perfectly. Each 3' strip comes with a perforation down the middle. You need to split each piece apart and flip the sections over so that the bevel side is to the outside of the rails like this /--\ . If you do not do this then it will be harder to lay your roadbed around curves and it will leave a square edge to each side of your rails which will not look realistic at all. You can leave the roadbed square if you are laying it in a yard or siding where you want to butt a few pieces together, otherwise lay the bevel edges to the outside.
Once you have the furthest half of the road bed in place do the same with the closest side. You will notice that as the carpenters glue is still wet you can slide the roadbed around a bit (unless you tack it). this is good and bad. It helps you line everything up but also allows you to in inadvertently lean on the roadbed and shift it out of place...take care not to lean on wet roadbed!
Repeat for the rest of your layout. It is a little time consuming but the finished product looks so much better with roadbed then without. You will really be happy with the results. After you lay 2 or 3 pieces you will figure out a few tricks to laying it straight and get a handle on how much glue to use and where you need to tack it in place till it dries.