Writing your own knitting pattern?

This is it! I have decided to man up! I'm writing my own knitting pattern!
Now of course, I have absolutely no idea how to go about this. It's not something simple, it's a shirt-type-thing. I've been looking and looking at patterns, and just haven't found anything that I really like. So last night I said to myself; "Rachel. C'mon, how hard can writing your own pattern be? Man up! Try something new! It'll be a challenge!"
So here I am. I decided that since I have no idea what I'm doing at all, I'll base it on something that I already own. I actually bought it at a real store! And it's not knit. But it's kind of what I've been thinking I'd like. Only there aren't patterns for it...At least not free ones. And why would I PAY for a pattern?! So yes. So far, I've done a TON of math. I then knitted my 'swatch'. Which was followed by MORE MATH. Oh yes, and somewhere between the math and the swatch knitting I completed my very sad attempt at drawing it out. Now, I've not mentioned this before, but I really, really, REALLY can't draw. So this thing is horrible. Not at all even. But it's covered in numbers and lines now, so I guess it's not so bad...this whole process is turning out to be a bit overwhelming. One minute I feel as though I have it all figured out, and the next I have something else pop into my head that still needs to be figured out. And again, I have no idea what I'm doing. Are you supposed to have it all figured out before you start, or do you just go with the flow, and kinda just figure it all out as you come to it?
Not that I've really cared what the 'right' way to do things is up until now. But this just seems a bit more...hmm...official? Real? I don't know. I can't describe it. I suppose I'll just make my own 'right' way to do it. Unless I fail. haha So far, it's seeming like it'll work though...I'm just hoping I've gotten the math all right so it fits. And I'm also hoping I can get the top shaping right. Because that's seeming a little bit complicated.
But hey! I'm making it myself, so unlike most patterns, it's knit in the round! Hooray for no seams!!