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Here's the nice thing about flats - you don't need to take them during the same session - just try not to change the focal length, aperture (focal ratio), focus or the camera's orientation to the lens (if you're able to rotate the camera independently to the lens).
Just pick up the camera and make sure it has the same settings you used to take astro images and start taking images of a white part of your screen. As long as the histogram is roughly centred (far from being black or white clipped) it'll be fine. Just keep it all square-on so the illumination is as even as possible. Move the camera around the white part of the screen (a big screen helps) and rotate the whole lens/camera setup too to break up any regular pattern that might make up the pixels of your screen (the pixels themselves will be massively out of focus, but the backlight or rectangular pattern of pixels could cause funny things to happen).
I have a stack of 50 flats I've taken that way and apply them to all my new images. Every few months I'll take a fresh set just in case dust or something has gotten into the light path.
Hope this helps. For camera lenses and small scopes I reckon this method of taking flats is a total breeze.
Cheers,
Cam
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