For long cage derailleurs, install the chain on front chainring and the largest cassette sprocket (without passing the chain through the derailleur) Measure the chain length needed as described in the groupset manual. Add 5-6 links so the last link is a narrow link.
For medium cage derailleurs*, install the chain on front chainring and the largest cassette sprocket (without passing the chain through the derailleur) Measure the chain length needed as described in the groupset manual. Add 3-4 links so the last link is a narrow link.
*for TRP medium cage derailleurs use the chain length as described in the groupset manual without adding extra links.