From the pattern:

This should fit premature baby to newborn size 7 pounds or less.
2 skeins of 3 oz Red Heart Baby Teri Yarn Worsted weight
Size 6 needles
2 stitch holders
3 buttons
Darning needle

Abbreviations:
K Knit
P Purl St Stitch
Rib K1, P1, repeat across Stockinette Stitch k1 row, p1 row
K2tog Knit 2 stitches together P2tog Purl 2 stitches together
Inc Increase BO Bind Off OS Other Side
YO Yarn Over RS Right Side WS Wrong Side

From the page the link goes to, select “Preemie Baby Pajamas” from the “Choose a Pattern” drop down menu at the bottom of the page.