The infinitive is used in Spanish in specific situations, generally when a command is directed to the general public through signs, for example in traffic, in towns, in public areas. It is also used when we are giving step-by-step instructions, for example in a recipe book, or when explaining how to build something.
If it is a negative command, you just place "No" in front of the infinitive. So there is no need to conjugate the verb.
Signs
Here are some examples:
Apagar las luces.
Switch off the lights.
No pisar el cesped.
Keep off the grass.
Step-by-step instructions
Doblar la hoja por la mitad.
Fold the paper in half.
Regar la planta con agua abundante.
Water the plant with plenty of water.