Construction help - mattress stitchhi

Hi all

I’ve had a prolific few weeks of knitting and now have a Cardigan and some wrist warmers to make up. I’ve watched the video for mattress stitch which looks fine but I have a few questions:

After casting off, should I continue straight into the mattress stitch with the same bit of yarn?

How do I secure the end when I finish, and secure the start if I’ve already cast off and cut the yarn?

For the wrist warmers, I need to leave thumb holes of around 4cm - what do I do with the yarn in that gap? If I just run it up the inside, I’ll keep catching it with my thumb which will be very annoying!

Thanks for any help or advice x

P.S. Just bought a stunning pattern book and some lovely alpaca yarn from Toft Alpaca Shop. It’s expensive but the wool is gorgeous and the designs amazing, so if you’re looking for something a bit special, I’d recommend it. Can’t wait to get some new 8mm needle tips for my 80cm circular set so I can try their moss stitch snood!

I just carry on right away with the mattress st or leave a very long tail so that I can do mattress st to seam. If you’ve already cut the yarn it’s ok, it just means that you’ll have an extra end to weave in.
With the yarn tail from the thumb hole, I would weave the end into the loops on the reverse side. Here’s a tutorial on weaving in ends that may help.

Thanks so much for replying - I’ll look into the video. Do you have to knot the yarn or fasten in some other way at the end of the mattress stitch before weaving in, so it doesn’t start to unravel?

Usually I secure the yarn at the beginning of a mattress st seam (most times that’s at the bottom edge of a sweater or the wrist end of a sleeve). I go into the cast on edge on the other side, back to the st that the yarn end comes out of and then put the yarn through the resulting loop and tighten. You could do the same at the finish of mattress st seams. I don’t know if it’s really necessary but that’s what I do.