#Crochet and #Sewing folks, I need your help.

I'm making a crochet pencil case. I'm planning to line it with some cotton fabric.

There are three separate panels (two in the images) to be sewn together. Should I line the crochet pieces before I sew them together or after?

I will be doing the work by hand as I don't have an easily accessible machine.

My sewing experience is limited to reattaching a button and making one or two wonky pouches, so assume I know next to nothing about it.

Depends what you're after. Crochet has a lot of give in it. You can retain the shape by attaching a backing piece, so to speak, and then assembling the pencil case. Or you can have a "free-hanging" lining that's carries the weight inside the case and is attached at the top.
I don't have tremendous sewing experience, so I can't advise on the zipper, if it's a zipper fastener you're after.
Attaching the zipper would be a significant consideration, I reckon.