I keep a basket containing yarn leftover from various projects and use it to make blankets. A knitted one I made nearly twenty years ago is still my go to winter blanket and is very warm. If you're using it under a comforter or quilt, it doesn't really matter what the colors are and it's a great way to use up yarn. If you prefer to crochet, granny squares are good too. I have made them with individual squares sewn together or sometimes, I'll just continue a square until it's bed size.
If you don't know how to crochet or knit look for volunteer groups who will teach you or look up You Tube videos.
Note: The granny square pictures aren't mine, but came up when I searched for granny squares.
Tip: If you have thinner yarn such as baby yarn, put two stands together otherwise your squares will end up different sizes.