Warm-up sets — why and how many
A light set before your working sets: it warms up muscles and joints and rehearses the movement with less weight.
Warming up to a working weight is a ladder: the empty bar, then 40%, 60% and 80% of your working weight for 5, 3 and 2 reps, plus 90% for a single before a heavy set. Few reps, short rests: the point is to get ready, not tired.
Warm-up sets don't count toward volume or records. I don't log them in the diary at all — light sets would skew your average weight, tonnage and "last time". When importing from Strong and Hevy I skip warm-ups but say in the report how many there were.
To build the ladder for your working weight, use the warm-up calculator; the what and why are in the guide on warming up for lifting.
Read next
In the diary I work out all of these numbers myself — you just log the set.