Sewed some parts of the pants together too a while ago.
First sewed the front and back together with the back having about .75 cm seam allowance and the front about 2 cm seam allowance, both sides were having their outside sides facing each other, this is the seam that will be on the inside of the leg
I forgot this step the first time, but check for long threads from the edges of the fabric that was possibly sewn as well, at this point their still easy to cut of and remove. Then I folded the bigger piece of seam allowance around the smaller piece and sewed that together
Then tidied the rough edges a bit by cutting, to prevent long pieces of thread to come out of the seam pocket I'm creating. After this I turned the fabric around and sewed the seam to the back part of the pants, meaning what is shown of the seam above is folded over to the left and sewn to the rest of the fabric. This encapsulates all the loose ends of the fabric. This seam finish is known as a flat felled seam
Then I finished this for the other pantleg too