How do I know when to use flaps in glide approach or force landing?
This is not so much for normal power-on approaches, but the proper way to manage a forced landing approach is to use some way other than flaps to vary your sink rate as required.  Gliders do it with spoilers, which allows the glider's descent angle to be varied between maximum glide and nearly straight down at any time, which is why the one-shot landing aspect of gliders is really not such a big deal; it's easy to place it anywhere you want.

With power planes you use a forward slip as your variable drag tool.  As you are getting in close, a few hundred feet, you drop just takeoff flap to lower the stall speed and apply some forward slip, ideally roughly half of the maximum you can apply, to be about what seems to give you the correct descent path.

It's as if you had some power.  If you are overshooting, increase the slip angle to descend steeper like you were reducing power, if undershooting, decrease the slip to descend less steep like you were increasing power (a little bit). 

Don't drop landing flap until you are down to the last hundred feet or so when making the field is assured.  Just about all aircraft can be slipped, but some may be restricted from slipping with full flap. 

If you use and practice this method you will get good at placing the airplane precisely into a forced landing spot without power, and a forced approach will lose its terror as long as there is an open place to land.  I'd be disappointed with an instructor who wasn't teaching slipping to control descent rate in a power off emergency.