Take some time to layout your entire design before applying your Decals. Use regular masking tape to hold individual pieces in place. This way you are making sure everything is where you want it before you start applying wall graphic.

When ready to apply make sure the decal is level.
Apply masking tape across the top of the decal you’re working with, making a ‘hinge’ with the wall. (Put a strip of masking tape across the decal 10-15″ below the top instead, if your decal is large).
Lift the decal up and peel the white paper backing off the decal. (On very large decals peel off only 10″ to 15″ from the top and fold that backing paper or cut it off).
Using squeegee, work from the center to the edge, than return to the center and work to the opposite edge. Firmly rub the entire decal, paying extra attention to the edges of each object and especially places where there are fine lines and small details. On textured surfaces rub very firmly!
If you still have any backing paper left you can peel it off and repeat the previous step.
Carefully peel off the sticky top layer (application tape) to reveal your graphic by starting from a corner and slowly peeling back tape at a 180° angle (parallel to the wall). Make sure your graphic stays on the wall as you’re slowly removing the tape.
When the entire graphic has been applied to the wall, re-squeegee the edges of the graphic to ensure a good bond.
Apply large decals in the order they are numbered.