Please help me, new to knitting

I am trying to knit some longies. I am a proficient crocheter but knitting is all new. I am having a hard time understanding what needs to be done here. Could anyone clarify this for me?

Pattern Courtesy of http://www.dunmani.co.uk/mrsflowerpot/FlowerpotTotsDKShortiesAndLongies.pdf
Longies Only
Using 3.25 mm needles cast on 50 (58, 66, 74) sts. Knit 6 rows garter stitch and continue in stocking stitch (starting with a knit row) until work measures 5 (7, 9, 10) inches, ending with a purl row. Make a second leg in the same way.
Shorties and Longies Gusset Shaping
Cast on 7 (8, 9, 10) sts. Knit 1st leg, cast on 12 (14, 16, 18) sts, knit 2nd leg, cast on 7 (8, 9, 10) sts. 126 (146, 166, 186) stitches.

My questions are as follows:

Do I cast off when I am done with the first leg and then start a new leg altogether?
How do I add these legs back when making the gusset? I am super confused about how this will look. I have the pattern but there are no picture tutorials of how this should be done.

Thank you in advance,

Sarah.

Welcome to the forum!
Don’t cast off the first leg. Leave it on the needle if you have enough room, move it to a spare or thread scrap yarn through the live sts.
Yes, start the 2nd leg as a new leg altogether.
For the join, you’ll have a section of new sts, leg one, a section of new sts, leg 2 and then a section of new sts. So for the smallest size it’s 7sts–leg 1–14sts–leg 2–7sts. There’s more shaping further on in the pattern but the 7+7sts at the ends will eventually join and make the front and back gussets the same size.

Thank you, that is helpful. One more question. When I begin casting on for the second leg do I cut ties with the first leg or just continue with the 1st leg moved to the back of the needle?

You can cut the yarn after knitting the first leg, leaving about a 6 inch tail to weave in later. The legs won’t be directly connected once the 2 legs are joined with the intervening cast on sts anyway.