Adapting a pattern

How do I adapt a pattern for #5 weight yarn - size 11 needles to worsted weight yarn and #8 or 9 needles?

It depends on the pattern. If it’s a scarf, hat or blanket, figure out what measurement the cast on sts measure and multiply that times the sts per inch for the worsted yarn and smaller needles.
If it’s a sweater, you’ll still have to find the sts per inch for your yarn and needles and see if one of the larger sizes will come out to a finished measurement that will fit.