For me it was studying how to let it happen, after achieving real kuzushi, in a slow fashion: thinking just of how to frame myself to pull as a single, heavy chunk in the right direction, using only bodyweight and leverage against the legs, raising the leg late.
As uke getting the "being pulled into a black hole" feeling as the force slowly and inexorably builds up, up to genuine breakfall.
That helped me immensely to get the concept, then it suddenly started working with combinations.
(Yes, I actually found nage no kata useful.)
Raising the leg late is good for tomoe-nage but not right for yoko-tomoe. Yoko-tomoe you need to plant the leg early and then drop your weight and use it to bring your opponent over and down. The advantage is you can adjust the direction of your drop to your opponents reaction and help get them over. The other big difference is that you need to keep your grip clamped on the lapel and arm and roll with your opponent rather than probably releasing and letting your opponent do a nice breakfall.
Yoko-tomoe is a much more down and dirty throw vs the nage-no-kata tomoe-nage.
Just as an update. My daughter just threw me with Tomoe Nage 3 times today. Still a bit unsure but that'll come with time.
Thanks for the advice (She got the Yoko Tomoe Nage in 1 day).