Decreasing while keeping track of the pattern

I’m shaping an armhole and I’m to caste off the first stitch of the following pattern k2tog, yfwd, k3… My question is, how do I caste off (and count) a k2tog? Also, how do you cast off the first stitch of a yfwd, k3…
Thanks ahead of time.

Generally - and especially with a lace pattern - you’d use/have/create a two stitch selvedge where the decreasing will happen. So your row would be k2, start wherever that lands you in the lace pattern, then the next time, you’d do cast-off 1, k1, start wherever that lands you in the lace pattern, repeat as needed.

Also a consideration with a lace pattern is that you want to ensure you keep the k2tog, yfwd pairing. So if you can’t do the k2tog at the beginning, don’t do the yo or your stitch count will be off. Just work straight knit stitches until the next repeat.

So it sounds like for this first cast-off row, you’d cast off one, then be in the k3 part of your stitch pattern, ignoring the k2tog, yo for this row. That would/should keep the row lined up with the ones below.

Wow. Thanks for the reply. It is a lot to digest, but I will give it a try tonight. I have tried so hard to figure this out.