Build a bit broader than you might imagine, trails naturally get narrower (usually) over time, and whilst a bike can ride something 6" wide when you start adding speed and bends it's nice (and more durable) to have some trail width to use (different people take different lines for the same bends etc).
Don't go too tight unless the trail is definitely going to be slow speed.
Berms are fine but there's an art and skill to riding corners well, try and keep some of them small and get plenty of flat ones in there.
Flagging tape and canes can really help when you're planning a line. It's fairly cheap. Pick it up when you're done though.
It's really easy to carry speed on even a gentle slope so long as riders get off the brakes. Braking and water are the two things that really erode a trail. Go shallow and avoid fall lines. Grade reversal is good.
Trail planning is usually a combo of squinting and thinking the trail could "kinda go there" (overall topography, maximising space and avoiding any no-go bits) then look for the features along the "kinda go there" line and do a bit of dot-to-dot to refine the line. All the while during both stages bearing in mind how much time, effort and materials it will take to build where you're thinking.
Oh, and TBH there's still a far bit of guess work / trial and error 😉
Work with the land rather than against it (how very zen ;-). Digging big drainage channels, berms and ramps can take a disproportionate amount of time and effort. Better to work with what you've got sympathetically. E.g. if the land already has boggy spots don't try and solve them, just avoid.
IMBA guidance is great but don't forget it's generic, don't be afraid to deviate a bit but admit when it goes wrong and learn from experience. Lee McCormacks book "Pump Track Nation" has some useful info on berms, rollers, spacing etc.