Proper belt tension is quite important and therefore here is the method I used for my Geeetech Al Prusa I3. The method is quite simple mainly because the Geetech quite surprisingly uses a properly designed idler pulley and adjustment system for X and Y (I replaced the not recommended smooth pulleys by toothed ones).
Here the pulley is held in a U shaped part with one screw in line with the belt used to hold and adjust it.
The method applies to similar designs. Note that it is quite surprising the wild contraptions some "designers" come up with for such simple things as an idler pulley with often separate adjustment mechanism leading to skewed pulleys, belt, hard to adjust tension ...
As can be seen, I replaced the nut by a butterfly nut, then made a hook for the scale using a piece of M3 threaded spacer plus a piece of threaded rod.
To adjust is simple, just attach a small electronic scale, pull on it till you read the required force then adjust the nut in such a way it just touch the frame, "That's all folks ! "
A proper tension is about 13N for a GT2 6mm wide therefore you have to adjust for around 26N (about 2.6kgf).
Note that if you machine is one of these crappy ones made out of acrylic sheets watch out that it may bend and even break so you now have an excuse to get rid of it :)