I know when I had to get the one on my left bar in I used vaseline jelly on the inside of the handlebar then tapped the end cap in lightly with a rubber hammer.
OP is asking how to get them in. I had such a hard time getting the ONE in on my XB that they're not worth it to me for the amount of protection they give. I'd much rather spend the $20 on the universal eBay slider type.