How to make your own tumblr theme?

I am sorta good with HTML, I can edit some stuff on the theme I currently have: Text size, spacing, color, captions/ titles if they show or not, and some other things, but I have absolutely no idea how to create my own tumblr theme. I don't have photoshop or dreamweave, so I can't use anything like that because I don't have them and don't know how to use them lol. Do you have any tutorials or can you tell me (in a simple-ish explanation) how to at least get started on one?