Wearing a garter belt and stockings effortlessly requires practice, but luckily it is not difficult to learn.
First of all make sure to choose the right size garter belt for your body. Too tight and it won't be comfortable, too loose and you will worry about the garter belt falling off. Most garter belt will contain elastane, and so will be slightly stretchy. Satin, silk and lace garter belts will not stretch, so make sure to get your measurements right before selecting your size. The best thing is to measure around you waist, just below the navel, as this is where the belt will sit.
Decide how many straps you want. Generally garter belts have between four and twelve straps;the more straps you have, the more supported you will feel. Six straps tend to be a popular choice for comfort and look.

Wear the garter belt on top of your panties and hook it at the back. Once fastened, the garter belt should sit comfortably just below your navel, with the straps hanging loose.
Now gently put your stockings on. Attach each clasp to the welt of your stocking, by placing the round part of the clasp under the stocking and then pressing the part with the loop over it, to lock it.
Once the clasp are all attached, adjust their lengths, lifting your legs to make sure that the back clasps will not snap.

Check out this 
video for an easy to follow guide on how to wear your garter belt!