Basically you just want the chain to run in as straight a line as possible from cog to cog...
Put the cog on the freehub body of the hub with no spacers. Now, put the chain on both cogs and spin the cranks backwards. The cog should seat itself in pretty much perfect alignment with the front cog ( helps reduce chain wear and reduce chance of skipping ). Now, look through your spacers and see what youve got, and try some different combinations to get the position of the cog as near as possible to the one it has seated itself in.
That should all be sorted now....slide the spacers on, then the cog, and the rest of the spacers. The lockring goes next which has to be tightened with this tool.
As for tensioners, I recommend using a spoke tensioner if your chain length is to within 1/4" of what is needed for perfect tension. Other wise I'd say a good 2-pulley tensioner or a homemade 74kingz style.
If you have any other issues, especially regarding tensioners, give me a shout
Alex