The best way to lear how to place protections is to climb sport routes that are also suited to protection placement. Bring plenty of quickdraws (and of gear to place of course).
Use the bolts, but place protections as if those were your only alternative up there.
This will, as the very first and foremost thing, allow you to place plenty of them. The more, the better. You will have a the real feel for that, and will be able to notice if they slip out of their socket when you move up the route.
Repeat again, plenty of times.
Next step is, place a protection just above a bolt, then FALL on that protection on purpose.
You will then be able to really tell if it would have held an accidental fall or not: while safely having the safety backup of the bolt just beneath it.
This will also teach you how to remove protections, which sometimes can be very tricky. Warning: it may take quite a long time until you get the hang of it.
Such is the way to learn how to trad climb. You trad climb alongside with bolts (eventually using fewer of them as you learn and feel more confident with your placements) so that you have the real experience of trad leading with the safety backup of bolts.