Please help! Two different yarns

I want to make christmas stockings for my nephews, and I would like to do a fair isle pattern or other decoration with contrasting yarn. My problem is that when I do this it tends to come out looking funny, some parts too loose and others too tight.

Any advice would be greatly appreciated!

Are you using the same yarn in just two different colors, or are you using two different types of yarn?

Are the yarns different weights? If so that would probably cause a problem. I think fair isle is best done with yarn of the same weight (both of them worsted or whatever). It can pull in even then if you aren’t careful about not pulling to tightly.

Yes, I do plan on using the same weight/type yarn. Should I try to knit looser or tighter to make it look better?

It’ll help to stretch the knitting on the right needle a bit so the floats aren’t pulled too tight. Knitting looser will help, too.