There are multiple ways of wearing your number in the jumper ring. These include wearing it on your back, attaching it to your saddle pad, attaching a small number to your martingale or breastplate, or attaching a small number to your bridle. The most popular is, by far, pinning it to your saddle pad.
Whether you choose to attach it horizontally or on an angle is up to you. Just make sure that the number is entirely legible!
To pin your number to your saddle pad, you will need a square pad and two or three large safety pins.
Instead, I like to use the pin to create an extra hole on each side. In order to create a nicer picture, I like to run the pins through from underneath so that the bulky parts of the safety pin are hidden beneath the number. When done this way, only a thin strip of metal is visible between the holes.
It will take some practice to get the correct amount of fabric on both sides to make the number completely secure. If you've done it correctly, the bulk of the safety pin should be under the number like in the photo below.
If you are worried about either of the side pins coming undone, you can run another safety pin through the top holes. Again, try to hide the pin behind the number. This time, you won't have to pierce any extra holes because the two at the top should be close enough together.