Find yourself a plain belt. You would probably have one lying in your wardrobe already. If the belts you already own do not seem fit for the purpose, just go out and buy yourself a relatively cheap belt. Going for an expensive belt would kill the whole point of decorating a belt at home, instead of purchasing a designer belt.
Before you start drawing, sticking or sewing different accessories on the belt, just sit down and plan the whole thing out. Think about the different creative designs that you can go for. Also consider the material(s) that you would require to make a certain design in your head a reality.
If you have decided to draw something on the belt, grab a pack of markers, crayons or paint and treat the belt as your canvas.
In case you have decided to accessorise your belt with faux gems, pay your local craft store a visit and try to get a good deal on faux flat-back gems. Go for gems of different colours and designs. Stick them on your belt using strong glue.
You can use buttons of different colours and styles to adorn your belt as well. Purchase a variety of buttons and sew them up on your belt using a needle and thread. In case you are not skilled at sewing, ask your mother, sister or some other female member of the house to do it for you.