Advice on fitting into the CB-6000 or similar lock?

I’ve been struggling a bit to get my husband into the CB-6000. Once he’s inside, it seems okay, but getting him in is tricky because of how narrow the tube is. It doesn’t help that he gets at least partially erect almost immediately whenever I bring up the device. Since I only received it last night, I haven’t had much time to try different methods yet. I’ve seen online suggestions about threading a stocking or ribbon through the tip opening, so I’ve been wrapping him with a ribbon and pulling it through. This works but can be somewhat painful, and I’m hoping for something faster and more comfortable. Has anyone here dealt with this before? What do you think is the best way to push his penis into that small plastic tube while he’s flaccid without causing too much discomfort? I’m still a beginner at all of this, so if there are any other tips I should know right now, please share them. I’d love your advice to help me avoid potential issues.