Thank you! Yeah, I do agree it can come off as a little confusing, especially if the people who have played haven't played kirby 64. Here's a step-by-step if it helps, and I'll also make an in-depth video demonstration after this post (There's also a mini example in the animated gif on the page of the game).
1. Hold the absorb button
2. Once you have an ability, hold the absorb button again until a triangle spawns above you.
3. Absorb a separate ability, then touch the triangle. After that, the two abilities will merge into a new one!
Please let me know if it works out!