The reverse kegels are a challenge by themselves, and then to do the stacking squeezes makes it that much harder. I am aware of this. It’s an area that we have not focused on much, but I personally have seen great benefit by incorporating it into my practice. If you can’t hold the whole time, that is OK.
But to the rest of your question, think of it this way. Gently push out as if trying to pee. I actually imagine in my mind I see not urine, but sexual energy flowing down from my bladder and out. This is a very gentle pushing out motion, not very hard. Maintain a slow steady breath.
Now the next part, the base. While holding the push out, that peeing sensation, gently push out as if to have a bowel movement or pass gas. You should first feel the muscles relax and sort of drop down. And as you continue to push gently, it will feel as if it opens up. The sensation will be more obvious at this point in the base than anywhere else.
You may feel a little diaphragm tension, but not too much. Breathing may be a little shorter or shallower, but it should not be difficult. Again, gentle pushing is key.
If muscles are getting sore right away, you are pushing out too hard.