Trim 101
the tools used for cutting and carving wood are inherently dangerous and extreame awareness and causion should be used
Find an area that is long enough to cut either end of the trim and set your miter box there .
Take a look at what's going to get sawdust all over it and move it outside maybe .
A hand miter box will work and I've even done it with a speed square and a hack saw ,  but a power miter box is the best way to cut trim .  Be very careful ,  this tool has no feeling towards you and will not be hurt by cutting off your arm hand or fingers .
when cutting the trim you may want to back cut the joint slightly by placing a drywall shim under the end of the board so when you put the trim on the wall the joint is tight on the front .  You will also need to increase the angle if the jamb is out of line from the wall .  It is often nessessary to test miter joints with scraps of wood to get the settings for the saw .