To begin the finishing touches, sew the top of welt to the top of pocket inside .(First picture is red to red, second picture gray to gray.
Then bring to other end of pocket to sew to the bottom of welt.
After the top and bottom are sewn then you'll sew the sides of the pockets
Sew from top to bottom of one side then the other and your pocket is done. (see pictures)
Then the pocket-less shirt is no more....