First of all are you going to paint of stain the moulding? If you are going to paint, buy paint grade moulding (about $.70 /linear foot). There is not reason to buy expensive clear moulding for a painted finish. When I install Crown or Cove moulding I start by determining which walls are the longest. I have a project for example, that has two walls that are 16' 6" long. For each wall, I would cut a 1' piece of scrap moulding with a 16' piece at a 45 degree angle. Cut both pieces in the saw at the same time (ends butted before you cut). Then I very carefull align them so they are flat and glue them together. When the glue dries, I double check the wall measurements and cut them at 90 degree angles Be sure not to assume each wall is the same length because the room may be out of square. When you attach the long strips to the wall it is critical that the bottom of the moulding is at the same height from the ceiling on the adjacent wall. For the two adjoining walls, if mistakes are made, it is cheaper to buy a short piece of you need to. Measure and cut the moulding with the back of the moulding facing you. and cut at a 45 degree angle to expose the profile. Also before cutting attach a straint wooden strip to the bed of your miter saw so that you have a fence for the material to rest against as you cut. This helps hold the back of the stock at a 90 degree angle for exact vertical alignment with each cut. If you try to hold it by hand, you cannot guarantee 90 degree positioning every time. This keeps the cut from becoming a compound angle cut. Now cut at a 45 degree angle to expose the profile. Use a riffler file and a coping saw to cut out the detail the riffler files are very fine and allow you to fine time the the cut of the file after making a rough cut with the coping saw. It is very important that the adjoining pieces do not fit tightly. You need a little bit of play for wood movement in the framing of the house. There can be enough movement in the walls with season changes to crack the moulding.