First put the piece on the bike (the gas tank for instance) that you will be installing the roundel on. You need to do this so you can see how the roundel will look and it will look different on the bike compared to if the gas tank is sitting on your work bench.
To place the tank and tail section roundels correctly, preferably first get some painter's tape, the green 3M tape from Home Depot because it comes off easily.
Then with a marker, place a dot at the exact center of the indentation where the roundel will be mounted (say on the tank).
Then put a long piece of tape horizontally right up against this mark. This tape is your horizontal reference point.
You want the tape to be long so you can stand back and look at it and see that it looks level and correct to your eye. You may also want to use a level to verify that it is level, but remember it must look "right" and "happy" to your eye, even if it is not exactly level.
When the tape looks correct, carefully tear out all the part of the tape that is in the indentation, this will give you working space to apply the emblem. This will leave you with tape only outside the indentation. That tape is your reference point for mounting the emblem horizontally.
Put a long piece of tape on the emblem horizontally.
Now you can place the emblem in the indentation dry, and match the horizontal lines of the tape and emblem (the blue and white 1/4 of the roundel) with the tape and know if it is level.
Glue or stick on the emblem (the new thin metallic ones will stick on instantly). If mounting an enamel one, you will want to glue it on and apply tape both horizontally and vertically to keep it in place. You can also ball up a piece of tape into a half or one inch ball, place this ball in the center of the emblem, and put tape tightly over this ball; this will put inward pressure on the emblem to help push it against the tank, as well as help keep the emblem from slipping while it dries.