These are a few methods of camouflaging with spraypaint. Remember not to spray to much, as this will cause runs and blemishes. It is a good idea to practice aiming on some card board or something before you start.
There are many deviations of the methods I will give, but they all follow the same concept of how they are done. Camo essentially consists of layers of colors in different patterns. in this case, each layer is applied using masks or stencils to create patterns.
It is also important to make sure paint doesn't get anywhere that could damage the gun. Use painters tape or masking tape to cover these parts if they cannot be removed. You should tape orange tips, joints, and holes that lead to the inside of the gun.

Step 1: Stencils

The method of using stencils involves putting something on the surface to keep paint off certain sections then spraying the entire surface with one color. They are not removed after each coat of paint. The colors will appear to be stacked in the order of application(eg. first color on top to last color on bottom)It is important to apply the stencils so that they are flush with the surface so that they do not let in any paint. Stencils can be applied either by spraying the back with some kind of adhesive or using tape.

Step 2: Makeing Stencils

Tiger stripe stencils are created simply by ripping both sides off a length of painters or masking tape to create a triangle. The length of your stencils depends on whether you want vertical or horizontal and how tall or wide your gun is.
To make digital stencils, use graph paper. Color in random sections of the "pixels" created by the graphlines and cut them out.
To make splotch stencils, draw blobs on some paper and cut them out.

Step 3: Tiger Stripe: Application

Tiger stripe stencils are made of tape, so no extra steps are needed to apply than other than actually putting them on the surface. Apply your stencils horizantally or verticly depending on preferance. The wide end of the stencil should start at an edge of the gun so it tapers away from said edge.
Digital and splotch stencils need to be attached with either adhesive spray applied to just thae stencil or tape loops. Spray is more exact but tape will adhere better to the gun and is less likely to leave residue.
So: spray your base coat, allow to dry. Apply stencils, Spray next coat, let dry, Apply stencils ect. ect. until you have sprayed all the colors that you want. After all the paint has dried on the last coat, remove the stencils and do the other side.

Step 4: Masks

This style of paintjob involves using foliage as masks to create natural designs. It is not as concise looking as stencil paintjobs, but can work just as well.
The term mask refers to something that covers part of the surface to block it from paint. it differs from a stencil in that it is removed after only the area immediately surrounding it is sprayed.
Use local foliage as masks. Spray layers of colors with masks randomly all over the surface until you are satisfied. you can let each spray dry if you want it really smooth, but you don't have to. This will be much faster than a stencil job.
The G36c in the title was done with this method. it required only three coats, two sticks and three leaves.
Once again, you do not give it complete coats, except for the base coat. you are only painting the area immediately surrounding whichever stencil you are using.

Step 5: Sponging

This method uses a sponge or sponges to apply the paint:

First, apply a base and a secondary color, then spray some other colors into a pan so that the paint accumulates as a liquid, and apply it to the gun via a sponge.

A really nice tutorial can be found here ( his guns turned out much better to).