After setting up Tanks for all the monsters, you will be brought to this screen:
From this screen, you can lay out how your Clerics should behave during combat. However, if you do not have any Clerics assigned as Healers in your Raid it will skip over this screen during the raid set-up. Initially, all healers are set to a default of healing the Most Injured Raider. This means that they will use their own best judgment when choosing targets for healing spells. You also have the option of assigning them to focus on Tank assigned raiders or a particular Tank.
You can use the Up and Down arrow keys to traverse the list of healers. You can then click on the Tank you would like them to focus their healing on, set them back to default (Most Injured Raider), or set them to focus on the Most Injured Tank. You can use the number keys next to the tanks as a shortcut, press the D key to set the currently selected healer to focus on the Most Injured Raider (Default), or press the M key to set the currently selected healer to focus on the Most Injured Tank. When a Healer is assigned to a Tank, they will get a number appended after their name that coincides with the Tank they will focus on & their name will be colored green. Healers set to Most Injured Raider will have their name colored red and those set to Most Injured Tank will have their name colored blue.
When you are happy with Healer assignments, click the Confirm button at the bottom of the screen (or press the Enter key) to continue on. The Back button, on the lower right side of the screen, will return you to the beginning of Tank Assignments.