Fix my fuckin bike, anons.

I started converting my 29" mtb to a single speed. 32:15 ratio. Thought I'd slap a chain on it, but I realized it was impossible to get it to be the right length.
So I'm using my old derailleur as a tensioner (only the top half of it, though)
But the problem is that the chain jumps once per revolution. Is this because the derailleur is aligned a tad wrong or if the chain is just fucked?