This blog post covers threading the adjustable punch needle using a 15cm long tapestry needle.
I've used a flimsy metal wire to thread my punch needle in the past. I much prefer using the tapestry needle - you don't have to fiddle with the wire, and you don't have to worry about the wire breaking.
Below is a step-by-step guide.
Step 1: Thread your tapestry needle with your yarn
Step 2: Insert the needle from the back of your punch needle, until it appears at the front
Step 3: Pull the needle out of the punch needle
Step 4: Thread your needle through the eye of you punch needle and pull the yarn through
Step 5: Once your punch needle is threaded, ensure you leave approximately ½ inch of tail of yarn.
There you have it! You're now ready to start punching.