I've seen Breaking Bad complete series 2-3 times, but I never found out how Walter White was able to poison Brock.
I mean did he himself directly feed the Valley of the Lilly berries to Brock or did he let Saul do the work? I can't figure it out.
Not everything needs to be shown, otherwise there's no suspense.
At the 2013 Comic-Con Vince Gilligan explained exactly how he and the writers imagined Walt got the poison to Brock, and it wasn’t, as I had thought, through Saul. Instead they pictured Walt as the “Evil Juice Box Man” going into Brock’s school and giving him a juice box that had juice from the poison berries.
And if you’ll look closely at episode 413 “Face Off” (thanks to Greg below for pointing this out), when Walt busts into Saul’s office, Francesca is shredding school schedules. It’s a big spreadsheet of classes and times but there are notes on the side that seem to be one student’s personal schedule, most likely Brock’s. So, kudos to the writers on including that little detail, and again to Greg for noticing that what’s was being shredded.