So it sounds like you didn't do this in the round so that it ends up like a hat without needing to sew it. In a lot of ways that would have made it harder to do. But you have gotten to the end anyway. So good for you.
I can't remember if you are the one who said they could take pictures and show them or not, but seeing what you've got could help us answer some of your questions. I'm wondering what you mean by saying you did the pattern backwards. And I'm not sure what you are referring to when you ask:
What would the difference look like?
Yeah, you can sew it up as Marria said, using the same yarn you knit with. Your first hat may not turn out perfect, but you no doubt learned a lot in the effort. And the next will be better.
This hat was designed to be working in the round, with no seam needed. Charts are also easier to read when done in the round. When you work back and forth you have to read the first row going right to left and the next row going left to right and keep alternating like that. Another thing when you work in the round the right side of the hat always faces you and if the chart says to knit a stitch you knit it and if it says a stitch is a purl you purl it. But working back and forth you have to adapt. When you are working on the right side of the hat you read the chart just as it is, but if you are working with the inside of the hat facing you when the stitch says it is a knit you need to purl it (so that it ends up being a knit on the outside of the hat) and when it says to purl it you have to knit it. It is a little tricky to keep you mind going right---at least for me. LOL
I think the hat said to work on double pointed needles, but I could be forgetting what it said. Hats are easier in the round as far as I'm concerned, but you would need some different needles. Knitting in the round isn't hard, but it helps to see it done. There are videos that show how to do it on this site. I believe it is under "advanced techniques", but don't let that scare you off. I taught my husband to knit and for his first project he made a hat in the round (of course I was on hand to help, but the videos can play that part). You can knit a hat in the round with a 16" circular and then switch to one of the next methods when you get to the top. These other methods are using 2 circulars, double pointed needles, and Magic Loop. All three of the last methods can be used to work the whole hat.
Hang in there, you will be a great knitter some day (and not that far off).