upcycling logistics

So, I love the idea of making kids clothes out of old clothes that I need to toss or give away and have stashed clothes away for this purpose, but I don't actually know how to go about doing it. For example, is finding the grain of the fabric to line up your pattern pieces difficult? I was wanting to make my daughter a pair of the after-school pants using old jeans, but not sure how to go about cutting out the pattern pieces. (How to determine grain line, how best to deconstruct the garment to get to flat fabric, etc.) Anyone willing to give this novice some tips?