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Drape the T shirt over the front of the scope with no creases. I hold it in place with a strip of elastic around the outside of the scope.
The scope needs to be aimed at a relatively even light source. This can be blue sky if you like but you need to keep the scope in the shade. If the sun is shining on the T shirt it risks a partial shadow and uneven illumination.
When I did this for a DSLR I just let the camera auto expose the image. It will be a very fast exposure and really doesn't need a dark frame to be subtracted. You might like to subtract an offset frame if that is part of you reduction process.
I use a different technique now as I have an observatory. I still use a T shirt but aim it at a white board on the inside of my observatory that is evenly illuminated either by overcast skies with the lid still on the observatory or 2 lights aimed at the wall.
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