The charities have a lot of information on their websites, below is some general advice to help you get those needles clicking.
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.
Knitting items for premature babies can be very worthwhile and rewarding. It is definitely appreciated by the charities and the families who receive them. If you are excited by the prospect of getting your knitting needles working for charity and want to explore charity knitting for other organisations here are some other ideas to help your search.
One of the first accessories that will become invaluable for you as a knitter is the knitting bag. This type of accessory does not only provide the perfect place to store your unfinished projects, your yarn and your knitting needles, but it will also allow you to transport your materials with you without worrying that they are going to become damaged.
The charities involved usually have free knitting patterns and advice on their websites so do not be worried by all the details as you really will be able to find a garment to suit both your knitting skills and the babies needs