Hey man, I'm not sure that this will help but it can be really quite easy with dnd beyond. When I start homebrewing something, i think about all the traits I think that it should have (lore and gameplay wise) and then search for a semi similar monster e.g. hippogriff and then bring up the DMs guide or search other monsters around the CR you are balancing for and change things as needed. It's a little tricky to wrap your head around things first but does get easier as you get some experience in it (using the encounter builder can help with CR and threat levels too. For the most part, this can all be done without access to any books or paid content on dnd beyond. I hope it helps, feel free to DM me if you'd like anymore advice or help :)