Echoing the comments above... practise, practise, practise.
It might surprise you based on my user name :), but for me the Rambone is now but a gentle warm up toy, I can basically insert it whole, from cold, and then go mow the lawn. But it's taken about 5 years to get to this, with playing maybe 1 or twice a week on average.
I started with years ago going deep using an 18" double header and lots of lube and lots of patience and, at times, a bit of pain. It's important to take your time and let those muscles learn to relax, there's the sigmoid but also sees to be another restriction at about 13" or so. Then it was a case of repeating the same process but with the Rambone. I can remember watching a video once of a fella getting one pushed in and watching it 'pop' through his sigmoid and thinking that if he could do it then it must be possible. So I just kept at it and eventually it did pop through and after that it was a little easier each time as you know it 'can' go it's just a case of relaxing into it.
Now days, a gentle 'session' for me is my home made 20" thick double ender. Or a BAM that I've trimmed the balls back a little so I can push it completely inside with the head right up through the sigmoid, so far that I can feel the head touching my sternum. I'll do some work around the house with that inserted.
An occasional deep stretch for me now is the King cock mega girthy, it's only 12" long so can only just get the head through the sigmoid , but the head is significantly bigger than the bam and the shaft gets much thicker than bam... it does take a good push and a bit of pain to get that head all the way into the sigmoid. Good pop when it goes though!
Just keep playing, using lube and try to learn to relax/stretch those muscles, watch some good porn to spur yourself on whilst you're pushing your limits... but don't totally ignore the pain either, if it's too sore, let it stretch a little, back it off, and then have another go. It could take months, it did for me, but if you want it enough you'll get there.