Buy the crown in the length you require plus 10 %. Always leave an extra ten % for cuts. If you make an error and have to go buy new crown, there’s no guarantee it will be from the same batch, or look the same when stained. I bought the crown in 14‘lengths because the room is big and I wanted as few scarf joints as I could get away with (scarf joint being where two pieces of wood meet, but not at a corner, along the straight away) Make this less noticeable by cutting the two pieces on a 45 degree angle to join.
I bought wooden crown for this wooden ceiling, you can also buy MDF, it’s become pricey almost as expensive as wood but its easier to paint it white if that’s your look. I stained the crown with min wax and a foam brush, let it sit a few minutes, and wiped it off with lint free cloth. Dry it for a few hours then coat again. Put it in the room where it is to be installed for 48 hours. The reasoning is to let it expand or contract before it’s on the wall, since if you nail it in and then it contracts or expands on the wall you’re left with cracked or bowed molding.
Measure the wall. Measure the wood. Cut it with a compound sliding mitre saw- that’s the easiest. Most regular chop saws won’t properly cut that size of wood. Remember that when cutting a piece of crown molding you have to turn it upside down, so that the bottom (the part that sits against the wall) faces up. On inside corners the bottom will always point to the corner. Seems simple, but this helped me out more than once in my career.
The little corners that seem to go back on each other is called a back reveal. That is easily done, buy figuring out the angles, and mitre sawing those little pieces.
All inside and outside corners are glued for maximum strength to prevent shrinkage and warping with weather and age. Use a nail gun to nail up the crown. Set in 1/16th of an inch. Fill all nail holes on the crown with wood filler the same colour as your stain.
Apply 2 coats clear sealant. Looks fantastic and really finished the room.