To add to Beartracker's info and let you in on another technique. I start by laying the gun on a sheet of paper. I trace around it, then carefully roll it onto the sights and over to its other side, and trace the other side. I then draw a line from just infront of the rear sight to 1/4"" behind the trigger. Draw a line approx. 3/4" around the outline of the the gun. You can draw the same distance around the bottom of the of the holster if you want a closed bottom, or just stop flush with the end of the barrel.
At this point I fold my pattern in half and match up my gun outlines, splitting the difference where neccessary. Cut out this pattern and transfer it to a piece of cardboard, I use a piece the approximate thickness of my leather, and cut this one out as well. Now you staple this cardboard pattern together along where you would stitch the holster. This allows you to check the fit of the holster without wasting expensive leather. I do this for the carry rigs I make so that I can check proper belt loop location as well. If stapled correctly you can wear it around a little and even do some draws to make sure it is what you want.
When everything is too your liking disassemble your cardboard holster and trace the pattern onto the backside of your leather. The cardboard pattern allows you to find the best location on your hide that wastes the least amount of leather. Rough cut and then trim. I check the fitt by wrapping the leather around my gun, then I offset 1/4" inside the edge for my stitches. Stitch on belt loop, then close up the holster. At this point I throw the edges of the holster at a drum sander or Dremel tool to even everyting up.
To mould the holster I run a sink of hot water (3" deep is enough) and add 5 drops of Joy dish soap. Submerge the holster for no longer than 15 seconds, I do ten, then remove and shake out the excess water. Lightly lube your gun , do not, do not, do not (got it) wrap in plastic. Place gun in holster and begin pressing the outline with only your thumbs. I will work a holster for about 15 mins. this way. Then remove gun and place some where to dry. No direct sunlight and no direct wind.
All that takes me about 2 hours, and an additional day to dry, and about 1 hour to apply finish.
That was a long post and I'm sorry, but there ya go.