I didn't. I only read up on the Pokes I like, and put them together.
I learned about the one's I didn't use from battling against them.
It really was trial and error. I use the tiers for whatever format I battle in, but I don't depend on others telling me what works, and what doesn't.
For example, I have a GBU triples team that is now 47W/12L. On the team is Jigglypuff, Amoonguss, Reuniclus, Swampert, Abomasnow, and Escavalier. The best, and most important Poke on the team is Jigglypuff.
It's running Protect/Substitute/Attract/Perish Song w/ a Calm nature, EV spread of 252 Def/252 SpDef/4 HP, holding Eviolite, and has Friend Guard ability.
No one on Smogon would ever suggest something like that for any team.