Chain run alignment is the key.

I converted a regular rear to SS. The biggie in the whole recipe is to flop the big ring over onto the inside of the crank spiders. From there you need a way to line up the sprocket you want to use with that run. A ruler laid across the face of the big ring (with no chain on yet) and long enough to nearly make it back to the rear axle will soon point out where the rear sprocket needs to be located. From there it's about finding a sprocket that'll fit at that location and be the right internal size for your freewheel. In my own case I used a freehub wheel and was able to butcher some worn out cassets that the LBS was throwning out for spacers to hold the sprocket in the right place.

And since you're likely working with a 52 or 53 front ring try a 17 rear. I found that this combo worked very well for my needs. And it still just barely lets me either speed coast or grunt up 4 parkade levels when I get to work. But it's just high enough that if I have to stop for traffic I need to get off and walk to the next level.