This pattern is available for free in Knitty First Fall 2016.
Here are some step-by-step instructions for making the welts:
Step 1: Using one of your smaller DPNs, slip needle knitwise into the purl bump 4 rows down to pick up this stitch.
Step 2: P into the next st on your working needle together with the st on your DPN.
Repeat Steps 1-2 for welt st.
How to make the box pleat: