There are a number of ways to start a project like this. You could start with a cell phone and a piece of leather, and wrap the phone up like a taco. Start cutting away everything that isn't a cell phone holster and Viola! you're done. We are going to use a slightly more formal process and start by making a pattern.
The picture shows averything you need, or at least everything I used. Your mileage may differ. The main part is a manila folder. There are professional pattern papers available, but I find the folder is cheap, easy to use, and lasts for a long time.
Along with the folder I use a triangle to establish a perpendicular line on the folder. This generally denotes the center of the pattern.
Next is a good pencil. It isn't necessary to use a fine point mechanical pencil. I have a friend who uses carpenter's pencils. We just need to make a strong line that can be erased, if necessary. A soft artist pencil works well.
Coffee. Have some coffee.
The french curve is a great tool for turning free-hand curves into proper curves you can use. Even if you can draw a good curve, use the french curve to neaten it up. It makes a noticable difference in the end.
The eraser is self-explanitory. Have some more coffee.
I like to use a snap blade utility knofe to cut out my pattern. Some folks use scissors, others use an Xacto knife. It is just a matter of preference, I suppose.
Start by using the triangle to draw a perpendicular line up the middle of the folder. We will use this for the center of the pattern. We will make the pattern by drawing the shape of one side and using that to trace the other side. That way both sides come out the same.
Lay your phone next to the line, about an inch up from the bottom of the folder. I like to stand the phone on its edge on the line and carefully lay it over so that it is next to the line, about half the thickness of the phone away. Then I draw the outline of the phone on the folder. The next step is to establish where the edge of the holster is going to be and draw that outline. We need to allow enough room between the phone and the edge of the pattern to allow for the thickness of the phone, the stitch line, the amount of leather outside of the stitch line, and the thickness of the leather. For a 1/2" thick cell phone, that comes out to be about 3/4". That might be a little much, but you can always trim the excess after you shape the leather. The last thing you want is to come up just a little short. Have some coffee and we'll move on.
So, draw that edge line, using your french curve to shape the bottom corner. Move the phone out of the way and draw where the top edge of the holster will be; usually the holster is shorter than the phone to allow you go grasp the phone and pull it from the holster. Once you have the outline where you like it, use the knife and cut out the outline from the center line, where the fold will be, around the edge line dn back to the fold line. You should now have a holster front-shaped flap of folder cut loose. Carefully fold the flap along the center line and trace it on the folder. Now cut this line and you should now have your basic holster shape cut out!
At this point you can fold your pattern around the phone and see how things fit. If the pattern is too big, you can always trim it down a little. Trim one side and trace a new line on the other side and cut that. Be sure to leave enough for the thickness of the leather.
That's enough for now. Next time, we will transfer our pattern to the leather, cut it and form and fit the holster to the phone. No more coffee. You've had enough for now. Grab a beer and we'll meet back here, later.