The charities have a lot of information on their websites, below is some general advice to help you get those needles clicking.
If you click on the resource link at the bottom of this article, you find hundreds of free patterns that will pique your interest. Under the image, Knitntyme specifies ”gauge/tension, number of stitches per 10 cm/4” or crochet hook. Suggested yarn may be replaced by yarn with a similar knitting tension.” So you don’t have to buy their yarn to download the pattern.
Bonnie Babies: This charity sends premature baby outfits and blankets to special care baby units around the UK and to parents who need support. A mother who needed a way to deal with the loss of her premature child founded Bonnie Babies six years ago. Bonnie Babies makes premature clothing, blankets, and burial outfits for U.K. Special Baby Care Units and families. Their aim is to show each mother and family that there are people thinking of them and caring for them.
Early Angels: This is a site based in the U.K set up to help people to knit, crochet or sew clothes, blankets and other keepsakes for premature, low birth weight and sadly stillborn babies. They have a wide range of free knitting, crochet and sewing patterns accessible on their pattern page.
Fastenings: Plain flat half-inch buttons are best to use for fastening. Avoid nylon and metal fastenings, as they get very hot under the incubator heaters. Do not use Velcro as a fastening, it is very scratchy on the hook side and also damages knitted garments when they are laundered. Ease of dressing is vital for preemies and as a general rule it is best not to use ribbon as a fastening as ribbon can often be fussy to tie on a garment so small.