Pour half of your gallon of flat paint into a large empty bucket.
Stir the paint well using a stir stick.
Add a half of a gallon of the semi-gloss paint into the bucket with your flat paint. Use a color of semi-gloss paint that matches as closely as possible to the flat paint you already have. This will ensure that the color you end up with is as close to your original choice as possible.
Stir the two paints together vigorously to thoroughly combine the two different types.
Brush the paint on a small section of one of the walls in the room you plan to paint.
Allow the paint to dry, and then inspect the sheen to determine if it is glossy enough for you.
Continue adding a small amount of the semi-gloss paint until you reach your desired sheen. Stir well after each addition. Once you have achieved your desired sheen, mix as much paint as you will need for the whole room. Plenty of paint will help ensure that each wall is a consistent color when compared to the other walls.