Good ventilation is key when keeping crickets. without it the small will build up and kill the crickets which in turn kills more. I use repashy bug burger to feed them but a variety of fresh greens and fruits that are safe are good to feed.
Make use of trimmings, scraps of human foods you prepare such as leafy salad greens, fresh fruit and veggie trimmings (not spinach or broccoli...they bind up calcium), plus fortified cereal grains. Some ingredients you may end up buying specifically (bee pollen or spirulina) to balance the nutrients over time. Sure there are great commercial gutloads offered by forums sponsors and the advantage is convenience, standardized recipes, and producing mixes in a form that keeps longer (shelf life).