A Japanese aircraft is probably the easiest to paint the national marking. It's just round and red. Right? Yes, that's true but cutting a perfect circle without some sort of mechanical help would be a special talent indeed. There are aftermarket masks and compass circle cutters out there but for a Hinomaru this is pretty easy and you are more than likely to have what you need on hand.
What you need
A piece of glass
Hobby knife with new #11 blade
*Any masking tape can be used but I like Tamiya because it is thin and translucent
Put a piece of tape on the glass that will be big enough to handle after it is cut. The glass is the best cutting surface for this because the blade glides smoothly. Draw 2 lines perpendicular to each other that bisect in the center, more or less.
... a perfect circle, perfectly positioned for a coat of paint.
All that's left to do is apply the mask to the model. The lines help align the mask using the panel lines on the model as reference points. Makes getting them in the exact same place on both sides a snap. Peel off the extra tape and...
You have an inner and outer circle and a choice. Paint the red 1st and mask using the inner part, or paint the camo 1st and mask for the Hinomaru. If you do the former take the tip of the blade to find the edge and peel the inner pieces one at a time. If you want to do the latter, remove the inside and, 1 layer at a time, put a piece of tape over the hole left in the tape. When you peel the 1st piece off the last bit of tape hold everything in shape.
Cut along the edge of the template being carefully not to cut into the plastic. If you are comfortable turning the knife in your hand as you cut, great. If you need to turn the glass to continue the cut, lift the blade before you adjust the glass. Go back a tiny bit to find the cut and carefully pout the blade back in to reduce the chance irregularities. When I actually made the cut I taped the template so it didn't move.
Place a second piece of tape, to cut 2 at a time, across the first. Draw the lines on the tape again using the lines below as a guide. Remember the bit about being translucent... ;o) The lines are to position the circle template for the cut to be as close to the middle as possible.
The outer part of the mask was used to first paint the Hinomura. The inner section was used to protect that from the camo application.