I just did this job a week ago. Clean the **** out of your inserts and also drag an acetone rag through the inside of the tube (long piece of parachute cord with a weight on the end to get it started. Debur the inserts really, really well.

As per the manual instructions, DO NOT FORCE the inserts in. If they don't go with just finger pressure and a push stick then stop and pull it out. The factory suggestion is a light coat of baby oil if needed to assist in the insert sliding in. I don't have a baby but I do have some LPS so that is what I used.

You'll get it, just take your time and STOP if it's not going well.

In my case I'm building up all new spars. Two of them went together easily and two didn't. I went over the mating surface with a scotch brite pad on the two that didn't go in. That, along with the deep clean and lubrication was all it took.