You can draft up a basic trouser pattern - or block - for yourself by using a series of measurements and dimensions, detailed here.
You need to trace the back and front trouser pattern twice on each of the corresponding sides of the dress, making sure they're traced out symmetrically so that both the right leg and the left leg fit correctly. Cut them out and sew them together along the seams as you would if you were making a normal pair of trousers.
If there aren't any fastenings in place, you might need to add them in order to get into your jumpsuit. I attached an invisible zip along the right side seam.
Cutting out the trouser legs along the side seam means that you don't need to include seam allowance or sew the outside seams together. However, cutting the trouser legs diagonally might leave you with excess fabric along the centre front, which you would need to take in for a snug fit.