Hi...
Obviously these can be altered based upon aircraft performance, but are typical methods I've used.
On a Non-Precision approach without step downs, gear down at the FAF.
Non-Precision with step downs, either the initial stepdown or the FAF, depending on performance.
Incidentally, a continuous descent is preferred during a non-precision approach.
Precision approach, glideslope intercept.
Again, these are not written in stone and Single Engine performance will dictate the best procedure for your specific situation.
Regards