Take off rear wheel and there will be spacers with your cog in the middle, take away the spacers and the cog, put the spacers on and then the cog, or the other way depending which way your chain needs moving. Chain wrap is were the chain wraps around the cog, like if you have a chain tensioner which pushes downwards only half your chain might be touching the rear cog, but if it pushed up, it would wrap around most of the cog, it needs to wrap about half the cog for it not to fall off all the time.