I'd suggest you first study up on reading spark plugs to make sure you actually need to re-jet:
http://www.google.com/search?source...lz=1T4DMUS_enUS221US222&q=reading+spark+plugs
And then if you need to go richer you'd either have to purchase some larger size jets or remove your original mainjet and take it down to a good hardware store that sells small drill bits called Number-bits. Find out what size Number bit just barely fits through your main jet orifice now, and then purchase the next 3 or 4 larger sizes. You can then use a pair of pliers or vice grips to hold the bit and then turn the jet with your fingers to easily drill it out to the next larger size.
You'd then reassemble everything, warm up your engine, go out and try to make a good full throttle run for 15 seconds or preferably longer, and then suddenly hit your kill switch coasting back into your shop. You could then do another plug reading to see if your still running too lean. If so, you'll want to keep repeating this procedure until your spark plug is giving you a correctly jetted reading.
Here's lots more info on re-jetting:
http://www.google.com/search?source...8&rlz=1T4DMUS_enUS221US222&q=rejetting+a+carb