The top that i'm currently working on for a client is one she loves so much that she wants it made in different fabrics (and I will post pics once it's finished). Once I have the pattern I can make it for her again and again, so it makes sense to do the pattern properly.
Stage one is very simple - draw around the seams and edges of your garment as closely as possible making sure you mark on where the darts start and finish. Then look at the inside and measure how wide the dart needs to be;
Fold in half and redraw making any uneven edges even and levelling lines up to match (You need to keep hold of this pattern for a bit later on);
This is how to add the dart: Draw the dart line in that you have copied from your existing pattern. Then draw a line from the end of that dart up into the neck line (or any other edge that you wish depending on how confident you are feeling);
Cut along the original dart line and then fold along the new line you have made, opening the dart up to the width it needs to be (mine is 4 cm);
Your dart is now back in your pattern but the neck line has changed.
What you need to do is redraw the bottom half of the pattern from the under arm down and the neck line down including your new dart. Then place your original pattern on top matching up the under arm and neck point and draw on the original neckline and shoulder;
This is your final pattern piece;
1. Draw around the top you want to make, measuring the width of the dart.
2. Fold it in half and redraw it marking on your dart line and a line to open up your dart.
3. Open up the dart by folding into the neck.
4. Redraw the bottom of the pattern with the dart in using the original pattern to redraw the neck line.
I really hope I haven't made this sound too complicated (all feedback welcome!).