The gore is just a big isoceles triangle of fabric (a base and two equal sides). This is how I would do it:
A. Decide how big you want the circumference of your hem to be when it's done
B. Measure the current circumference of the hem
C. Decide how many gores you want to put in - for example, if you decide on six that would be two in the front, two in the back and one in each side seam
D. Calculate (B - A)/C. That will give you how long the base of each triangle should be. If you are picky about measurement or are going to use lots of gores, you will have to take into account seam allowances. Then the calcuation is (B - A + 2x(#of gores NOT going into a seam)xseam allowance))/C.
E. Measure from the hem to your knee (or whereever you want the gore to start). This is the height of the triangle.
F. On your pattern paper, draw a line for the base (the length from D). At the mid-point of the base, draw a line perpendicular that is the length from E. Draw lines connecting the top of this line to each corner of the base. There's your basic triangle. From this, don't forget to add a seam allowance at the top and sides and add some on the bottom for the hem (I'd add at least 2" for the hem - always easier to shorten than lengthen, and you may have to trim the edge to get the right curvature).
G. Make equally spaced slits (and/or split seams) around the dress - same number as number of gores. Definitely a situation to measure twice (if not three times!) and cut once! Just a suggestion - I'd try gores in the side seams first to see how it looks before I actually cut the dress to accommodate the other gores.
H. Pin gore into slit. Sew up one side, across the top (you will not have a perfect point, but should be close) down the other side. Once you are happy with how it looks, sew back and forth a little bit (.5" or less) across the narrow end of the triangle to strengthen it - there's a lot of stress on that point. Repeat for every gore.
If looking at a pattern will give you more confidence, the Simplicity Jack Sparrow pattern, oops! I mean, "generic pirate costume" (anyway, #4923) has gores in the coat. But they're really not that hard to do! Good luck!