In order to heal your covenmates you need to switch to the healing mode. Click "Heal Coven" in the bottom left corner of the battle screen to switch to the healing mode.
You can see your Avatar on the left and the Avatars of your covenmates available for healing on the right. The healing mode uses the same "Match 3" mechanics: you heal your covenmates by making a match of tiles. Every swap in healing mode costs 50 energy. Your covenmates become available for healing when their health drops below 25% of their maximum health. When their health reaches 75% of their maximum health they become unavailable for healing.
You can choose whom you want to heal. To do that click on one of your allies' avatars. It will become highlighted and the other avatars will become transparent. To choose another ally to heal, simply click on their avatar. If you don't choose anyone, healing will be split randomly between all available allies.
What influences the healing efficiency?
Base healing: by default you heal 500 health for your ally with 1 swap. If you have the “Battle Healer” skill upgraded, the healing value is increased by a certain percent (specified in the description of the skill). Additional matches and tiles also increase the amount of health you heal for your allies.
There are special boosts that will help you heal more effectively. They work the same as damage boosts, except they add a healing bonus instead of a damage bonus.
Please note, that damage boosts don't work in healing mode and the other way around.
You can also get a special tactical task - “Mass heal”. It works similar to a special attack. It doesn't require energy to use. It's also 10 times as effective as the base heal.
How to get the rewards for healing?
To receive the rewards for healing ranks you need to heal a certain amount of health for your allies specified in the rank's information. When you reach that amount the rank is considered completed. For example, if you heal your ally for 550 health, you get 550 points towards your healing ranks. So it's better to use all resources available to achieve higher ranks faster and get better rewards.