Make the paint test on a moveable and repositionable surface. Something that can be affixed temporarily to any wall, ceiling, or even floor (when you are painting a floor). Note: do not use your blue tape on the edges of this! Tape the back side only..
Make the colored surfaces in proportion to each other, for example: Trim colors will usually be narrower than the wall color surfaces.
Be sure the surface you paint the test on is primed, or at least under-painted with a color similar to what you’re testing. For example, you can of course use poster board but since it’s paper, it must be oil-primed so it does not warp. Personally I like the flat-finish, fast drying low odor products for this. Note: I’ve typically prepared up to 20 or so poster boards with this type of oil primer, to have ready to go in advance–and easy to use at a moment’s notice–when preparing color tests for clients.
Roll on 2 coats of your finish (test) color, making the application as close to what you intend to do on the final surface. Use a roller with similar pile to what you plan to use on the job.
Use a stable surface with a texture similar to the wall or surface to be painted. IF the target surface is textured, it’s worth it to make a replica since color reacts so differently with the light falling on textured or smooth surfaces.
Be sure to do the test with the same finish you’ll be using on the actual surface (Flat? Eggshell? Semi-or-high-gloss?)
Label your sample cards and cut a nice, smooth-edged one about 8”x8” to leave with the client for their own ‘shopping’ purposes.. I find that people appreciate this especially when they are looking at window treatments and furnishings.