Taking measurements, designing 3D printed brace: The next step is to take accurate measurements of the arm you are designing the device for, and using those to design the 3D printed braces. Initially, we went a little overboard by getting a super accurate 3D scan of my daughter's arm using a Kinect. This was because we wanted to print out a perfectly fitted brace. Later we found a way around this by printing the arm brace flat and molding it to the arm after it is printed. This is what I'll first describe below. I'll include the instructions for creating a detailed scan below that as optional. 

Measure the following:
Measure the circumference of the forearm, upper arm and their length. You will use these measurements in the next step to tweak the design we already have. 

Note: The arm brace will be worn over clothing, therefore make sure that the measurements are not too tight and that you include some breathing room. You will notice that once you mold the 3D printed braces they can be tweaked easily by adding heat from a hair dryer to the area of the brace you want to tweak. This helps to make a really good fit, even if your measurements are not 100% accurate. 
Design the Braces: 

Now that you have the measurements it's time for you to alter the braces to fit your use. Below we have the 3d files that can be printed using a 3D Printer. If your arm measurements are similar to ours and you need a left arm brace then I would use the files as is. Otherwise, you will need to use software like Solidworks to tweak it.

We ended up designing the brace flat. Below are three files. The upper arm is the brace that fits around the upper arm. The forearm is the brace that fits around the forearm, and the combined is simply a file that includes both braces (This combined version was easier for us to print at Shapeways). We then used a service like Shapeways to print out the file for us and ship it to our home address. This was really simple and meant we did not need to purchase a 3D printer. 
Note: It was really useful to print out the design on paper first to test it out. You can tape it to the arm and use 2 drawing pins to pin the two braces together at the joints. You can then move the arm, making sure it does not pinch the skin and that there is enough space for movement. Solidworks allows you to print out flat paper versions to scale. I don't know if other 3D design programs can do so.

If you want to go for the accurate 3D scan of the arm and print our the brace to exact measurements then you could use the Kinect like we did. Initially, we had challenges with this as we needed to keep the arm in a single access with as little lateral movement as possible as we scanned. We overcame this by using a plate on which she would stand while we scan. 

For the Scan you will need a Kinect, an adapter to connect it to your PC . Then you will need to run something like Microsoft 3D builder. Here is a video that may be helpful: https://youtu.be/nXbDkJ7cRrA

Step 1 - Understanding the problem
  Step 3 - Measurements & 3D designing