If you're making your white trousers from scratch you can cut the white lining with the same pattern of your trousers, put it over the fabric and sew all the layers at once.
If you already have a pair of white trousers
Option n°1 Take them inside out, stretch well the fabric flat and sew the lining on the seam allowance along the edges of the the front and the back panels of the trousers (stitches won't be visible on the right side of the fabric).Cut a rectangular piece of lining, pin it along the seams and cut the excess, no pattern needed. Be sure the lining is not tight and follow the seams. Cut two pieces of lining for the front and two for the back for a better fit. The length of the lining is up to you, you can make it at knee length, but the "shorts" length is perfectly fine.
Option n°2 Make a pair of shorts made of lining and sew them at the waist of the trousers or keep them detached and wear them under every light colored pair of trousers you have.
Now wear your white trousers with confidence.
|the lining sewn with the trouser fabric pic via seh kelly|
|men trousers inside out showing the lining - pic via a tailored suit|