How can I gut-load meal worms and crickets for my leopard gecko?

I feed my leo mostly mealies and sometimes crickets and want to know how I can gut-load the insects besides just the stuff the mealies come with and the cricket food from the pet store. I heard that pet store food can kill the insects and thought I would make my own. Would oats, fruits, and veggies work well?

Thanks!!
P.S.—Sorry about all the topics I really want a quick answer