To turn a granny square solid you do add stitches in some of the gaps, yes. You can find instructions for solid granny squares here: https://lucykatecrochet.com/solid-granny-square

Hi Elizabeth, The pattern instructions show how to produce the circular central shapes and then turn them into squares. Do you mean you’d like to know how to attach the squares together? If so, this article should help you out! https://lucykatecrochet.com/how-to-crochet-a-blanket-with-multiple-granny-squares