Paint passes through a stencil's openings. The material left behind holds the design together. This requires a different drawing from a logo you want to cut out as a separate shape.
Which pieces remain attached?
Look at the design as a sheet with holes. The centre of an O would fall out without a connection. Add one or more bridges to hold that piece. The same applies to a logo with enclosed areas.
Bridges appear as interruptions in the painted image. Position them deliberately, for example along an existing line in the letter. They must not be so narrow that they tear during cleaning or handling.
Keep an outer border
Leave space around the artwork to handle and secure the stencil. Cutting very close to the lettering can make film difficult to hold. A large design may benefit from support or splitting into several parts.
Use consistent reference points
Alignment openings or a fixed corner help repeat placement. Use the same references for each stencil in a multi-colour design. Distinguish paint openings from positioning marks in the file.
Test with the actual paint and surface
Try the finest details, bridges and fixing method. Check whether the stencil sits flat and paint seeps underneath. Clean the trial to see whether small pieces remain intact.
Mylar film is flexible; PETG can suit a stiffer template. Choose for the intended use and check the actual sheet properties.