Oh jeezy... Right, I'll keep this simple so forgive me if it sounds patronising...
First, get someone to straddle (NOT sit on) the bike and hold it upright.
Ideally, you'd want to loosen the bolts holding the rear wheel axle in place, but I'll assume you don't have the tools to hand. You'd also need the bike lifted off the ground, but that'll be awkward without a centrestand or a bike jack.
Make sure the chain is straight and securely seated on both sprockets (the cog things with the teeth).
Then look along the swingarms (oblong things out the back, with the rear wheel at the end).
Coming out the back of each swingarm, there will be threaded bolts with two nuts on.
First, you need to loosen the rearmost nut right off (turn anti-clockwise). Use a pair of spanners, or maybe one spanner and some pliers. Hold the inner one tightly in place and turn the outer.
Next, tighten the inner nut a couple of turns (clockwise). As you do, you'll see a little block inside the swingarm move and the wheel will shift backwards a bit.
If you examine the swingarm, there will be little graduation marks, that look something like this: ||||||||
Use those to note how far you move the wheel backward.
Now, be sure you tighten each side equally. Best to only do a couple of turns at a time and then do the other the same amount, rather than many turns on one side.
Continue until your chain has about an inch (2.5cm) of slack at the loosest point. Turn the rear wheel round a bit (roll the bike forward/backward if you have to) and re-check the tension. It may need a little alteration on the adjusters until you get a fairly uniform slack measure.
Once you're happy with the chain slack, tighten up both outer nuts to keep it all in place and you should be good.
Also, see section 6-22 of the Owner's Manual (page 53 of 78), which can be opened in PDF here: http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDAQFjAA&url=http%3A%2F%2Fwww.ymenvom.nl%2Fnew%2Fmanuals%2FU5JXE3.PDF&ei=AFySUZb4L8n40gWmpoGICw&usg=AFQjCNGAd7UtLCqtGw4mmhpmxBoX-GkeVw&sig2=CQd1Xq8slM9Hsz5JP3eHfQ