It helps to know the basics first.
When you've mastered things like meditation, grounding/centering, opening and closing magic circles, moon phases, planetary associations, reagent associations, chakras and the like, you're ready to start casting more advanced spells. You can learn about them through the Newbie tips on this site, books written by experienced practitioners, or Google search - though be warned, there's a LOT of misinformation on the Internet about how to do it. Use your common sense when determining if a source is credible.
Frankly, learning about the basics teaches you everything you need to know about drafting your own spells. The more you read, record and perform, the more obvious spell tropes will become to you. You can adopt these tropes and add your own twist to them, or you can invent your own.
Ultimately, spell writing is an intuitive and imprecise art. So long as the intention is there and you've learned how to harness it, any ritual, reagent or tool can become a conduit of your will.
Good luck, and blessed be.