This happened last night.
I cast on with the needles that the pattern suggested but I am knitting too loose. This afternoon I started again and the tension is still a touch loose but much better.
Why are there two needles? You start with a provisional cast on. Andrea has you do a really cool version where you cast on to a second needle so when it’s time to use the stitches they are ready to go. No picking out waste yarn. I used a 100cm (40”) needle for the cast on and the saddle shoulder is being knit on a 60cm (24”) needle. Because of the different lengths it is easy to figure out which needle to use.
Here is Andrea's video which is very easy to follow. Grab two circular needles, scrap yarn and give it a try.
Now a few more rows before bed. It’s going to be a busy week.

