The main point of a ghillie suit is to break up your outline. Camo clothes work alright for helping you blend into the background, but they can't hide your silhouette like a ghillie suit does.
Step 1: Buy Camoflauge Stuff
First, you need to buy a pair of camo pants and a camo jacket.
While you're at it, you can buy a roll of camo material.
Keep in mind the kind of terrain you will be operating in, and choose the pattern accordingly.
Step 2: Cut It Up
Next, you want to cut the material into thin strips. These do not have to be straight, and wavy edges might be even better to help break up your outline. These will go on your helmet and clothes.
Step 3: Cover Your Clothes
Now you'll glue the strips to your helmet and clothes. Only glue the top part of the strips, as you want them to hang loosely to break up your outline.
I'd recommend Gorilla glue for this, but hot glue also works.
Step 4: Cover Your Gun
Get some brown or green rope, tie some strips along it, and wrap it around your gun.
Other people like to use camo tape, but this is how the US military does it, so it must work.