I would honestly like to know how that's even remotely possible. Are you talking about modifying it, bringing your knee up to his head and putting your shin under it and grabbing your foot over his head while putting your other leg over his back (Think Kimura from bottom position)? That seems next to impossible against a good ggrappler as he should never give you the space to get the leg under his belly and over his back.
Also, this is technically from guard, not side :P
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