The gauge of your pattern determines which weight of yarn you should select. You can use any yarn that has a suggested gauge equal to the gauge of your pattern. For a list of Lion Brand yarns by weight, click here.
This method works best if the gauge of your pattern is given over stockinette if knitting, or over single crochet if crocheting.
Be sure to swatch and check your gauge carefully, using whatever size hook or needle is needed to obtain the gauge of the pattern. Be sure you are satisfied with the fabric that results; it should be neither too stiff nor too loose.
For more tips on yarn substitution, check out our blog post here.