You join Tread Lightly at Tread Lightly - Home
. The cost is $25 for a year membership. They send out the membership packets on Fridays. In this packet will be your member number and a redemption code. These numbers will be needed when you fill out your mail in rebate for the Goodyear tires.
Speaking of that, the rebate is only good on select Goodyear tires. These include MT-R Kevlars, SilentArmors, and Duratracs. The rebate is for $100 and the rebate form and information can be found here
So, all told, you are looking at $75 in savings, once you factor in the cost of the membership itself.
Another thing to keep in mind is that DTD also has a $60 MIR for select Goodyear tires this month. Add this on to the $50 discount forum members get when purchasing 5 tires and you can get a pretty substantial savings (~$185).