This is a method of increasing your Restoration skill in Skyrim easily and at no cost. You can do this for as long as you want to get to whatever level you want.
First you need to travel to the location of the Atronach Stone on the world map and receive its blessing. The location of the Atronach Stone can be seen on the map below.
The Atronach Stone gives the character a large magicka boost (50 extra points) and enables them to absorb incoming spell damage (50% extra absorbing) at the cost of a reduction to their rate of magicka regeneration (50% regeneration decrease).
When you’ve received the blessing of the Atronach Stone, travel to High Hrothgar, home of the Greybeards. Make your way to the temple courtyard and over to the archway beyond which lies the path leading to the very top of the mountain, to which your way is blocked by an ice storm effect that damages you when you move close to it.
Equip a self-effecting Restoration spell in one hand and walk up the steps to the archway and step into the harmful area of effect while activating your restoration spell at the same time and keeping it active while you remain in the area of effect. You will take damage from the ice effect, and you’re active Restoration spell will heal the damage as it is inflicted. As soon as you run out of magika or your health drops dangerously low, step back from the area of effect and wait until they are both fully regenerated. All the while you’re using your restoration spell to heal the incoming damage, your Restoration skill will be improving.
The incoming damage from the ice effect is classed as magic damage, and because you now have the blessing of the Atronach Stone, you can absorb 50% of magic damage, which allows you to spend a longer amount of time in the area of effect before you have to step away and recover. This extra amount of time really stacks up as you repeat this process many times, allowing you to get to your desired Restoration skill level quicker than would be possible if you had to step away from the ice effect to recover more frequently.
Make sure that you only use one hand to perform a Restoration spell when you do this; don’t use a spell in both hands. Doing so will drain your magika too quickly. The overall process is more efficient if you use one hand to perform the healing magic.
Also, if you have a follower with you, you might want to tell them to wait at the foot of the steps leading up to the archway as they will sometimes step into the area of effect themselves and eventually die when players do this.