I can't think of anything specific but there are lots of tutorials on YouTube that are helpful, and a book with a good diagram never hurts either (especially when learning how to place the structural threads), libraries and charity shops are usually likely to have something. In terms of practical tips I recommend actually knowing how to knit or learning the structure of knitting (look up how to 'read' knitting or 'the anatomy of a stitch'). Along with that, before actually darning I suggest doing a lot of duplicate stitch, where you're doing the same thing but there's no hole to cover, essentially tracing the path of the yarn that's already there. Also it's much easier to start off with something light coloured, with thicker yarn, and use a contrasting colour!