Make some sample test pieces, 'til you get familiar with how to use the tool, to obtain ''closed'' joints (no gaps). Whether using hand tools or power tools.
Clamp the molding to the fixture/back gauge, or the like, before making any cuts.
A slight movement of the piece, while cutting, will result in an open joint.
Make the cuts, so the chip-out edges are away from the finished surface/side.
A slight bevel on the far side can be made---so the ''toe'' is closed and the ''heel'' is slightly open.
The toe being the outside joint and the heel being the inside of the joint.
(the heel joint can be open slightly by 1- 2 degrees, and will not be visible from the outside.
It just takes practice to make perfect crown molding closed joints.
Install a "stop block strip", (1x or suitable cross section member), on the horizontal surface---
(akin to a ceiling).
Position the stop strip, so the inside surface of the crown molding touches the stop, and the top surface edge of the molding is flush.
The stop strip will suffice as a spacer, and provide a means of affixing the top area of the molding to the stop strip.
Make a dry fit and tack the pieces in place first, so as to allow for small incremental adjustments, to obtain a tight fit.
Tack in place before final fastening.
Install the outside corner joints first---it's easier to obtain to "closed" joint.
Any imperfections can be filled-in and less noticeable at inside corners.
Place the first brads away from the end, so as to avoid splitting.
(Brads to close to the end will result in splitting).
The use of a pin nailer will produce the least visible holes to fill.
For smaller size work pieces, a ''repositionable adhesive'' is useful.
For some applications, I use ''tacky putty", available from office supply stores.
Alternate: ''stop block strip''
make a "triangular backerboard strip"---
positioned so it fits the inside corner, and the sloped surface to butt to the back-of the crown molding.
(Prevents the crown molding from shifting during installation).
You could look up the DIY Network online. That has some idea in how to cut molding and also look at other online videos like on you tube. You could buy some molding and make your own by mixing and matching different types of molding. DIY Network could show you how to do that also.