Restoration is very useful to have a high level in because you will quite often find yourself in the need of healing and using a regeneration potion all the time is expensive and wasteful. This guide will help you easily reach a high level in restoration quickly.
First you will need to find a group of enemies. I would suggest that you do not use enemy mages because they can be quite powerful and unpredictable and eventually run out of magicka and then attack you with their weak dagger. Once you have found a group of enemies (around 5 or 6 is a manageable number) you can either turn the difficulty up or you can take off all your armor. Either one will increase the damage you take and thus can heal.
It would be good if you have a few potions of magicka ready so that you can continue this process for a while, or try to tweak the difficulty or the armor you wear so that you can heal any damage you can take without needing a potion. The latter would be the cost effective way, but it will take you longer than if you take the maximum possible damage.
The master trainer of restoration is Danica Pure-Spring who can be located in Whiterun in the Temple of Kynareth. However, in order for her to train you you have to complete a quest for her.
Simply go to the place marked on your map to retrieve the Nettleblade. There are quite a few mages and a Hagraven there so come prepared.
To make the roots of the large tree move strike it with the Nettleblade. Once you reach the tree you will have to interact with it. Some enemies will spawn that are somewhat tough to kill. Now you can return to Danica Pure-Spring and she will train you in restoration up to level 90.