My method for attaching in seam side pockets without a pocket extension.
Lay pocket piece right side down on the garment side piece, matching waistline and side seams. Sew together within the seam allowance. My seam allowance is 1.5cm so the pocket was sewn on at 1cm from edge.
Fold pocket away from garment and press with seam towards the pocket and edge stitch on the pocket flap through the seam allowance as close to the seam as possible.
Repeat for all pieces. As I was doing a pocket in each seam I now had four side pieces.
This is a close up of the edge stitching through the pocket flap (sorry upside down).
With right sides together match front and back side pieces and pin. I draw my stitching line in chalk.
Then stitch from the waist down the side seam to the top of the pocket opening, backstitch to reinforce. Then stitch around the pocket and down the side seam at the full seam allowance (mine was 1.5cm). The pocket seam will now be enclosed within the pocket.
Repeat with other pocket and press seams to the front. Finish the raw edges in your favoured method.
Inseam pocket from outside complete.
Continue the rest of your garment.
This will work reasonably well on most things which are not very close fitting. If the fabric is particularly bulky the best thing is to put extensions on the side seam and make the pocket pieces out if a much lighter weight fabric.
Have fun putting pockets in your garments now. Don't be afraid to try different things too.
I hope this made sense.