There are many different ways to do that. Here are some of them.
The first is to paint that area in the color of your stripes, let that dry, then apply very thin strips of tape around the bomb. Paint your bomb color, and remove the tape. A variation of this one is to apply liquid mask for the stripes area, rotate the bomb against a knife blade, peel off the mask, and then spray your paint.
Another is to use "tubes of tape", you start with the nose color, push the nose into your tube of tape, paint the stripe color, let dry, do bomb color, do this alternating until you have the final yellow stripe on, then paint the bomb body color.
You can use very fine tape if you want to, just apply the stripe with tape, and clear coat over that after you have all the stripes on your bomb.
Some decal companies make pre-curved decals for the bomb stripes.
And, if you are building in the larger scales, you can paint them on freehand. Edit: If you are painting freehand, you might lay your bomb down on two dowels, letting the fins stick out over the end, then you can hold the brush steady, and rotate the bomb under the brush,,,,,,this might be more steady than trying to move the brush around the bomb.
I hope this helped,
Rex