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
Step 2: Makeing Stencils
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.