All you need is this.
http://www.jejamescycles.co.uk/onza-single-speed-item124146.html
this
http://www.tartybikes.co.uk/sprung_tensioners/4jeri_tensioner/c48p10179.html
and to shorten your chain, that is all.
If your chain is not wide enough go and buy a single speed specific one.
Stage one:
Remove cassette lock ring, this requires 2 tools, a lock ring tool and a chain whip. Internet guides will show you how.
video guide below:
the cassette should just slide of the hub,
Put spacers and new sprocket on the hub, rearrange them until the sprocket is in a straight line to the crank sprocket.
put cassette lock-ring back on the hub.
Shorten chain, put back onto bike with the tentioner, ride!
If the chain is to long you wont get enough chain wrap and the hub will skip, same with bad chain line.