First, you'd need to decide how big you want it. Find the size needles you want to use, maybe an 11 or 13? It goes quickly and uses less yarn than smaller needles. You'll have to find your gauge per inch with the yarn and your choice of needles. Decide how wide you want it and cast on the required number of stitches. If your gauge is 4 stitches per inch and you want a blanket that is 40 inches wide, you would cast on 160 stitches, for instance. Knit and measure until you get to the length you want.
My two favorite patterns are the twin rib stitch. This is a multiple of three. Right side rows are k3, p3 across. Wrong side rows k1,p1. This is like a rib but flatter. The edges don't curl up. This makes a blanket that is reversible.
My other favorite pattern is the thermal knit stitch. Cast on in multiples of four. First row, knit. Second row, purl. Rows 3 and 4: k2,p2 across. Repeat these four rows for the pattern. This gives you a knit like on long underwear. This blanket is very warm, but can also be heavy in weight.