its pretty complicated, you have to take your leg like your going for hald guard with 1 leg but then immediately go around and up the back and to the shoulder for gogoplata position, wrestlers can be pretty stupid with top control. i sparred with probably the best bjj person in the gym and i got tooled trying to have an active guard and pull off subs and as a result i got passed pretty easy
Maybe I am stupid but a gogoplata from side is impossible.
The way he describes it it goes as Side Control -> Transition to modified rubber guard -> gogoplata I do believe. So it's technically not a gogoplata from side control, it's a transition to it from side control though... thought I'm still not entirely clear on the "Leg up like you're going to half guard then over the shoulder....etc" part. To me it sounds like if they're on your right side you would use your left leg to go over their shoulder and secure rubber guard, but then you have no entrapping leg on their right side... so they could easily just spin out. If they're inexperienced grapplers though it may be possible as a gogo is a very "HOLY SHIT I'M DYING" submission. It's one of those where, if you're not used to it, you want out ASAP.
Also, if you get your leg over their back like that, why not just post for a sweep/stand up or take their back?