One little tip: i had an old bike with stem shifters (eventually replaced them with bar top shifters). Anyway, this isnt exacly stem shift related, but on older bikes especially, you can run into an issue called "cross chaining". This is where you try to shift the front and back cogs onto opposite sides (example: shifting onto the outside ring in front and inside ring in back) entering into the large/large or small/small cog combo. You'll notice that it can be very tricky to shift into a cross chained position....on my old bike I absolutely can not move to the large chainring (outside front) with any amount of force if I'm already in the large cassette cog (inside back)

Anyway, its an easy fix...just don't try to get in those positions. You don't even really need them, since the large/large and small/small gear combinations are somewhat redundant because you'll always be able to find a slightly higher or lower gear.