For Em/D, Am/C etc straight Em and Am will be fine.
Well, you're right that, just because the note is on the "bottom" of the slash, that doesn't necessarily mean it has to be on the lowest note of the chord voicing you play on the ukulele. The uke just doesn't have the range to make that practical in many cases.
But ideally, the note should be played
somewhere.
You don't have to worry about the C in the Am/C because an Am chord already has a C note in it. (And if you play it as 2000, it actually is the lowest note!)
Same goes for Em/B. The B note is already in an Em chord. If you play it as 0432, it's not the lowest note, but it's still there (on your first string).
Ah, but the Em/D is different. A D note is
not normally in a Em chord. You'd have to convert it to an Em7 (0202, for examle) to get that D note in there.
So, to recap: If the note on the bottom of the slash is already in the chord on top, you can ignore it. If not, you should try to figure out how to add it in, but it can go anywhere.
JJ
P.S. Half the battle with these chords is simply figuring out what they're called. Once you know they're "slash chords", you can search UU and Google for tons of info.