Pattern question

Hello, I am knitting a cardy for my daughter and am having difficulty understanding the pattern so any help is greatly appreciated!
My pattern gives me instructions for the first 8 rows then says:
From 1st to 8th row sets slip st rib and cable panel.
Keeping continuity of slip st rib and cable panel as set (throughout) work 21 rows dec 1 st at each end of 5th and every foll 8th row.

My question is how do I know what stitches to knit for the next 21 rows? Do I repeat the last 2 rows? If so it’s going to get very complicated with the decreases.

I’m a bit of a novice so I hope I haven’t been to ambitious with my pattern choice!

Thank you in advance

Repeat the 8 row rib and cable pattern (probably mostly rib with the cable crossover every so many rows). The decreases will take away stitches from the rib pattern (and may eventually eat into the cable pattern). That’s okay, just continue the rib with one fewer st on each end. So instead of k2p2k2p2k2 for example, you may have k1p2k2p2k1 for a few rows and then p2k2p2 after the next decrease as you lose edge sts. You want the columns of rib to continue up the sweater.
Can you give us a pattern link or the name of the sweater?

Thank you! I started with 88 stitches then in row 3 there is a decrease at either end so I now have 86 stitches so shall I disregard the first stitch and the last stitch in rows 1-3 (although row 3 starts with P2tog???). I’m guessing it will become clear as I start to knit and make sure I follow the rib.
The pattern is from a Sirdar book, Little stars in stripes. 355, code C if that helps. I could scan and email the pattern if you would like?

It sounds like you have the right idea: follow the rib. You should work the decreases (p2tog if that’s the start of row 3) and then continue to work that edge st on succeeding rows. It won’t fit into the [I]original[/I] pattern but that’s ok and it’s the only way you can decrease and still keep the main columns of stitches aligned. Have fun knitting this for your daughter!

Cables are like ribbing except for the rows where you cross the stitches, so if you work the sts as they appear - knit the knits, purl the purls - at the edges as you decrease then you’ll stay in pattern.