Others will said don't try it, it's impossible or hard but just from what I've done with my pair, it was VERY VERY easy.
Here's what I did:
I have a 3" GSM in a 20 gallon tank. I bought a medium size plastic breeding container from Petsmart. I then order a small GSM clown from the internet. When small GSM clown arrived, I acclimate it and put it in plastic breeding container to see how aggresive my larger GSM clown would be toward it, surprisingly, there was no sign of aggression.
I remove the small GSM clown from container and drop it into the same space as my larger GSM clown. Within seconds, the small GSM was doing the wiggling dance to the larger GSM clown whenever the larger clown trys to nip/bite it. Since the smaller clown yielded completely to the larger clown, there was no fighting at all, no torn fins or anything.
After 10-15 minutes, the small GSM clown was following and shadowing the larger GSM clown like they've been together forever.
It's been 2 months now and they both are getting along great. They both host in my green long tentacle anemone.
So basically like other's have said in many other posts, if you are going to try to pair your GSM clown, make sure you get another one that is 1/3 or less the size of your existing GSM clown.
Hope this helps ya.
Beng