I am having a hell of time getting my feet jammed well into vertical finger cracks. Do well if the crack is on a lesser grade, but trying to take it up a notch and go for the more verticals but can't get my toes to hold. Can't even get toes in crack. Tried starting at little toe and rolling it in then turning the knee up, and where I can move up on it, can't hold it to get next jam with fingers. Just slip. Smearing, with big toe kind of in the crack works best, but it puts a lot of weight on the fingers and not making it very far, and really want to be able to jam it. Any advice would be amazing.
I'm no crack expert, but often the feet go outside the finger crack. Look for stems. smears and edges for the feet. If your toes won't even fit in the crack, maybe you're looking in the wrong spot for the feet.
If you are talking a splitter with little or no feet outside the crack, you need to learn what I've always heard of as the rand smear.
It's the pinky of your foot in a very froggy type position. You need your feet much higher than you would dare if there were edges or if you could fit your feet in. It puts a lot of force on your fingers so you have to be strong enough to do it, it's not a technique that makes hard climbing easy. It makes hard cracks not campusing.
In places like IC, look for itty bitty edges on the crack or the pod from blow out cams. Elsewhere, the edges will be bigger and you might piton scars. AIM FOR THESE. Put your rand up to your belly if you have to. Let your overly tight hip flexors hold you in.
It takes practice but it works.
And get pointy, flat, and soft shoes. They'll suck for everything else. My lowest end shoes (Evolve Defy and before that, Evolve Quest) have like 99% of the hardest cracks I've done, up to 13a but a lot in the mid to upper 12 range. Both of those shoes are pretty much gym rental shoes with a pointier toe than average.
Other crazy advice, get super strong at sport while working through the cracks. The extra strength makes a huge difference that is hard to develop by actually climbing cracks.