In your home directory just type mkiraf and respond to the prompt with xterm . This will create a file login.cl and two sub-directories, uparm and images , for you. You should make sure that you run IRAF from an xterm window, else the display may prove erratic. The nature of IRAF IRAF is hierarchical in nature with packages , tasks , and parameters .
Packages contain tasks or further packages, and tasks contain parameters.
As the name suggests it is the tasks that will perform some operation on an image or spectrum, the exact operation being determined by the way you set the parameters for that task. When running a task you will be prompted for several parameters if you don't specify them in the command line. You should be aware that IRAF stores the last values for all parameters that you used for a given task, and will use these as the default the next time the task is run. At the most basic level, the pre-processing can be achieved by using the one task images.imarith , where images is a package name, and imarith the task name. This task allows you to add, subtract, divide, and multiply images together by setting various parameter values.