I have actually gone about it several ways.
I have done the base coat of Yellow and lightly drew on it with a pencil (if I am recreating a specific scheme or tank) and I have freehanded from looking at pictures or just a pattern of my own. You can't mess it up, it's your own creation.
Most, if not all, WWll German Armor has a blended look to it. There's not too much of a "Hard Break" like you would see on modern armor (Nato)
If you do wish to mask:
I have seen silly putty used for masking after the base coat has been applied and I have also read articles where Parafilm M has been used. It streches easily and does not allow bleed under like some tapes do. It also releases easily with NO residue left behind.
Good Luck!