I'm not sure how I'd put a wheel in a horizontal dropout alone, especially without a frame.
I've done it many times, but I'm not sure exactly what I do.
Same procedure as for vertical dropouts (set shifter for top gear, line the small sprocket with the top run of chain, pull derailleur back and guide rim between brake calipers) then shove it all the way back till it meets the stop on the derailleur side, now centre the tyre between chainstays and tighten QR.
Something like that. It only takes seconds.
Granted it would be tricky if your bike did not have a frame.