It depends on where you're riding and what type of rider you are. Loud Thunder begs for a lower gear than Sylvan. Some people mash, others spin.

That being said, a 2:1 ratio is a good place to start for 26" wheels. You probably have a 32 tooth middle ring, so use a 16 tooth cog. Ride it, see what you think. I'm riding a 34/18 on my Bonty (both SS and Fixed), and am converting the 29er to SS using 32/18, but I tend to go for a little lower gear than most SS. (BTW, rule of thumb for 29" - add two teeth to cog to match 26" ratio, so a 32/18 29" is similar to 32/16 26")

As far as how to go about it, that depends on what components you have. I assume if you are converting your Yukon, you plan to use the existing parts. If so, all you need is a chain tensioner and spacers with a cog splined for a cassette hub. Gusset makes a good gear/spacer kit; Surly's singlulater is a decent tensioner.

Rg.