Looking for advice on designing crochet waistcoat/jacket

[HenriettaMaria]

I have a notion to crochet a granny square jacket, waistcoat or pullover and I get on better designing knitwear myself than I do following someone else's pattern (truth be told, I've only done it once, a Donegal-type sweater, but it worked).  Does anyone have any pointers to books or blogs that explain the process of designing such a thing and calculating the yarn requirements?  Thank you 

[BrendaP]

You need to start with a schematic - a drawing of the pieces with the measurements included. 

Crochet doesn't generally stretch as much as knitting so you might be able to adapt an existing sewing pattern.

Then you need to make a granny square or two, measure carefully and then do the maths.