Best way I have found is as follows:

Make stencils/or tape up

Then to keep this simple, say you want to apply red on base white

Once the stencils/tape are in place - in the area you want the colour to go, seal the 'gap' where the paint bleed is by applying the base colour white along the edge of the stencil/tape.

This will then seal the area for paint bleed.

Have fun