Judo: Opponent Gives Up His Back From Standing: What Should I Do?

Hey guys! I'm a Judo noob. I've done a bit of JJJ and a bit of BJJ in the past so my (very limited) game right now consists of basic hipthrows and basic newaza.

Last night, during randori, one of my training partners consistently gave up his back from the standing position. He grips my lapel and sleeve and then it's like he overturns over me when attempting to off-balance me and I almost always wind up to his side or behind him and able to reach around his neck to attempt a rear naked choke. Every time this happens, my sensei (who knows I've done a bit of JJJ/BJJ) tells me to take him to the ground rather than attempt a standing RNR but I'm not sure how to safely do this. I could yank on him or just quickly sit down and try to get my hooks in but that sounds kindof of risky to my partner. Any thoughts on how to accomplish this? (Or, if there's something more judo-ey I should do from this position, I'm all ears!)

I've managed to get a few guys into guillotine choke from standing, too. Same deal here. Sensei wants me to take it to the ground but I'm not sure how to do it safely.

Thanks for the advice!