I'll try to answer this as best as I can without confusing you, then one day I think I'll write a hub about it.
1. The first thing to do is to sketch the dress as best as you can on paper, I know this can be hard especially if you have some difficulty drawing, but try your best.
2. Take your measurements - of your neck width, bust, waist, hips, arm width and length.
3. Now this is the tricky part, if you've sewn with a pattern before it will be a little easier but if you haven't it will be more of a challenge.
Each part of the dress will need a pattern piece.
4. Starting with the top of the dress, draw out the pieces individually on paper, use brown paper which you can buy as a roll or the thin white paper you put in gift bags.
5. Of course the pieces have to fit your measurements.
If you have an old dress, take it apart and see how the individual pieces look. That will give you some idea as to how to make a pattern.