Making slitmasks is a painful, but unfortunately necessary, part of observing with LRIS is Multi-Object Spectrograph (MOS) mode. At present, the most trusted program to design LRIS slitmasks is Autoslit v3.20.
A guide to its installation can be found here.
Requirements for Autoslit include:
You can find Autoslit's user manual here or in the location Autoslit was installed.