First, using the Mun or Minmus for a slingshot is a TERRIBLE idea and doesn’t worth it . we are doing this for science dude.
Like in first experiment, we want to intercept the Mun periapsis when Mun->Minmus is at a phase angle of -90.49°. For this we will use Kerbal alarm clock. The manoeuver node tells us we need 1d 1 hour and 50 minutes to go to Mun periapsis. We set up an alarm 1d 1 hour and 50 minutes before Mun-> Minmus is at phase angle of 90,5°.
Window is at y1 d7 h5 m49
Alarm is set at y1 d6 h3 m30 (with a margin of half an hour)
Now, at Mun periapsis, we tweak another node to encounter Minmus with a minimal dv of around 80m/s
And finally, we do a small mid course adjust to match the plane of Minmus.
So now, it’s clear that with numbers, it is possible and in fact, almost easy. Next time we are going interplanetary and test the famous KEM (kerbin -> Eve -> Moho.)