Step 1: Join the outseam

Join the outseam (the seam that runs along the side of your leg) of the front and back.

Repeat the step for the other leg, but make sure they are the mirror image of each other.

Step 2: Join the inseam

Join the inseam (the seam the runs along the inside of your legs) of the front and back.

You now have a leg. Repeat this step for the other legs, once again making certain they are mirror images of each other.

Step 3: Join the cross seam

Turn one leg with the good side in, and the other with the good side out.

Now tuch the good side out into the good side in leg, so that they have their good sides togetherSee Good sides together in the Sewing documentation.

You can now align and sew the cross seam.

You probably want to insert a zipper

You should insert a zipper or leave the last part of the cross seam open (so you can pin it).
If you don't you wont be able to put on Titan to check the fit.