The charities have a lot of information on their websites, below is some general advice to help you get those needles clicking.
Bonnie Babies mainly makes blankets for premature babies. They provide patterns for toys, hats, and sweaters (including a ”5 Hour Baby Sweater!”), but blankets are quick, simple to make, and guaranteed to fit. Families can then continue to treasure them long after the child is grown.
Garn Studio allows the visitor to print free patterns in eleven different languages. There’s even a link for American English patterns and British English patterns. You’ll find many crochet and knitting patterns for children and babies: hats, sweaters, blankets. You’ll also see links for their yarn on the Knitntyme.com site at discounted prices, however, you can use your own yarn.
Size: Don’t put too much importance on trying to perfect the size of a preemie item of clothing. There is a great difference between premature baby sizes and so an item of any size should be suitable for at least one baby. As a general guide premature babies head circumference is roughly the same as their chest circumference. The average premature babies chest measurements are 8”-14”. However clothes of all sizes are needed for preemies, so no matter how big or small your item is it will most likely fit at least one baby.
Feed the Children: Feed The Children is a UK charity that has a knitting pattern for a jumper appropriate for children of all ages. They are also look for knitted hats, gloves and scarves.