Tig's method can also be done using a crochette hook. Slip the ring over the hook then hook the leader loops off the peg and slide the ring onto that leg of the leader.
Another way is to twist and furl the leader. Put the Shorb loop in the butt end. Then, again with the crochette hook with ring on it, pick up the small loop at the tippet end and slide the ring onto the leader. Then put the knit picker through the small loop and latch the Shorp loop pulling it through and sliding the small loop all the way down the leader to trap the ring.
A 2 mm ring requires a #14 crochette hook and a 2.5 mm ring can be done with a #11.
If you are using 2 mm rings and furling 4# mono the method discribed by Tig works the best as there is too much bulk/od at the tippet end to get the ring onto the leader.
The 2 mm rings work well with both methods on 6/0 Uni tying thread or .005 C&C nylon.