Note: I am sure that a very clever prop designer created the blanket found on Ron Weasley's bed in the Harry Potter films. I am not that prop designer; I am merely a fan who had a lot of random yarn to use up. I was asked by a user on Ravelry to create a pattern page others could link to when making their own Ron Weasley blankets.
A whole bunch of yarn in various exciting colors that knits to about 5 sts/10 rows per inch in garter stitch.
I would also suggest finding one or two good screenshots from the films to get a feel for the blanket. If you can't make these yourself, The Leaky Cauldron has a great screenshot database.
Guidelines for squares:
I found 7 inch squares looked the most authentic to me.
You'll want to knit some of your squares solid. The patterned squares should be striped patterns only, not stitch patterns. This is a garter stitch blanket.
Knit a few squares on the diagonal.
Include some heathered yarns, or some tweedy yarns.
Guidelines for finishing:
The squares in the original blanket are arranged so that the garter ridges are aligned alternating vertically and horizontally. Basically, you will always be stitching a cast-on or bind-off edge of one square to a row edge of another. Except for the diagonal squares, obviously.