New to socks- working the heel flap- help!

Hello! I hope someone is awake to help!
I am brand new to knitting socks. I am making top down socks and started on 4 double point needles. I am at the point where I need to start the heel flap.
My instructions had me divide my stitches 16 on needle 1, 8 on needle 2 and 8 on needle three
then it tells me to:
“knit across ndl 1, heel flap will be worked on ndls 2/3
row 1: *sl 1, k1, repeat from * across.
row 2: *sl 1, p1, repeat from * across.
work these two rows until you have 16 heel row flaps. heel flap should be close to square. end having worked a purl row.”

This is my question. I knitted the 16 (on needle 1), then sl1, k1 on needles 2 and 3. Now I am back to needle 1. What do I do? Do I knit, then on needles 2/3, sl1, p1? Or do I sl1, p1 all three needles?

Thanks in advance!

For the heel flap, you are only going to work on needles 2/3. That means that you won’t be knitting in the round for the heel flap but knitting back and forth on needles 2/3. Let the sts on needle 1 rest there for the moment.
You’ve done the sl1, k1 on needles 2/3 so now turn the work as you would at the end of a row, and work back sl1, p1 on needle 3 and then needle 2 (for the back side of the heel flap). Do this for 16 rows total. Essentially, you’re treating needles 2/3 as one straight needle. Good luck with your first socks!