This set of fingerings is a great template to use with left hand warm up exercises. I say template because there are tons of ways you can add extra difficulty and variation to them. Rhythm, dynamics, articulations and right hand fingerings will all change the workout you get.
The basics are fairly simple. By playing through the set of fingering as seen below, you will have exhausted every combination of fingers available to you. Unless you have an extra finger, in which case I am very jealous. The fingering follows a pattern and once you get used to it, you will be able to easily run through the exercise without notes in front of you.
When doing the left hand warm up, focus on your technique and your sound. Your left hand fingers should stay nice and close to the string and use the minimum amount of movement possible. I like to think of engine pistons, with their exact movements and controlled repetition. In your right hand you can focus on creating a consistent, full sound.
As you get more adventurous, you can start adding variety to the exercise to challenge yourself. Try dotted rhythms, accents, different right hand fingerings etc. A full set of variations can be found in: 5 Technical Routines for the Classical Guitar (Intermediate/Advanced)