I'm getting back to KSP after ~6 years, so I only did the primarly challenge this time.
The ship: I did a regular 2-stage ship for this one (first stage achieves a circular orbit around kerbin and the second stage transfer to and lands on Minmus. To carry 20 kerbins I used a 4 seat pod and 2 8 seat cabins.
Liftoff: harder than I expected (I actually iterated over 10 times on the ship design as it was super wobbly and sometimes unstable when trying to rotate during climb)
Shaping the orbit: Circularizing the orbit around Kerbin was a challenge for this ship (it has a ton of inertia and does not rotate easily when commanded to). Once the first stage is left, things start to ease off.
Transferring to Minmus: somewhat easy maneuver after the orbit angles are corrected and the first stage is ditched.
Fairings out: once the ship got into the orbit of Minmus I finally decided to remove the front fairing. I could have done this sooner, I guess.
Completely unrelated to the challenge, I absolutely loved this view
Landing process begins: i truly thought this was going to be the easiest part but boy was I wrong. I took forever to find a somewhat flat surface and had to settle for a slightly inclined one.
Finally landed: like I said, I gave up on finding a flat surface (as seen by some legs not actually touching the ground). The building-like architecture is tough to land. I might try a more horizontal approach later.
Also clearly I overestimated the amount of fuel needed for this mission. On previous iterations I ended up having too little fuel for landing, but apparently I overcorrected.
All 20 kerbals are safe on their new Minmus base