It’s really difficult to get right. Creating a sphere and filling it up is not that hard, but when you start moving it the whole thing becomes quite involved. Not sure if it’s all that doable without a partner to be honest.
I’ve rolled a bit in my wing chun training but I’m quite awful to be honest. The only things I could suggest at my stage is to just keep the sphere structure like you’re holding a beach ball in between your arms, then fill that structure by expanding. That has helped me a fair bit.
All the good people I’ve rolled with have really heavy arms and you can’t move them. All the really good people have those heavy arms but if you push against them it feels like moving through air as they don’t directly take your force. These things are good feedback when practicing with a partner and I’m unsure how you’d even know you’re doing it right if you’re just by yourself.