You need a source of camouflage diagrams - such as those sold by the Floating Drydock or in some of the popular press books on the subject.
You also need a Xerox machine capable of shrinking & enlarging copies
Copy the diagrams at the appropriate scale. Cut apart & use these as paper masks when painting. Apply with Blu-Tak or double side tape.
Start with your base color - apply that. Apply the lightest color mask. Paint the next lightest color. Apply that mask leaving the others in place. Repeat for each color of paint.
When all the paint has been applied remove the masks & touch up.
You can use plain paper as your mask or you can run photo frisket film through the X-rox machine to make sticky masks