I cut antler either at my bandsaw (with dust collection running) or with a hand saw (cheap modern dozuki usually).
I lay the pen tubes against the antler section(s) I think I want to use to see if the antler is straight enough and wide enough for each tube.
Every tube is held in at least two positions...the second position is 90 degrees from the first one (rotate the antler 90 degrees). Antler curves and flows in multiple axes so it's important to check that your tube fits from multiple directions.
I love the tips for letter openers, bottle openers, iPad styluses, etc.
Dust collection is critical (as others have mentioned).