Noir Et Blanc Vie (a Keyboardist/Producer/Videographer from Los Angeles) has some super helpful beginners guides to starting a Eurorack system, along with a whole YouTube Channel of other goodies.
Eurorack for Beginners Series - Part 1 of 3 - The First Things You'll Need
Eurorack for Beginners Series - Part 2 of 3 - Smart Module Selection
