How to Sew...

Flat Felled Seam:

With Right sides together, sew along the edge of the fabric with at least a 1/2" seam allowance.

Trim one of the seam allowances at about 1/8" from the stitching line.

Press the seam flat. Then, Open the seam and press it open.

Fold in half the biggest seam allowance towards the stitching line and press. 

Fold again over the trimmed seam allowance. Press to keep it in place.

Top stitch over the folded edge at about 1/16" from it.

With this seam, you get a decorative stitch on the good side of the garment.

This is a reinforced seam, great for inseams and side seams of pants.

For the full video Tutorial:

https://www.youtube.com/watch?v=8G0mRBlAP58