Oooh good luck. I draw on iArtbook. It’s free to use, but if you want to use like rulers or insert references you have to pay for it (I have the free version). To get the upgraded version it’s the same price as procreate, which is really good. There’s more features on procreate and I’d say it’s better, but if you just need a free temporary app, iArtbook may suffice.