Here is a link with complete instructions, along with tips to reduce your power usage when you are using solar panels/battery power. Cost is about $50 to convert a chest freezer to a fridge. This was posted elsewhere in the forum too, but I bookmarked the actual page and can't find the other post.
Convert Chest Freezer To Fridge
Btw- you want a chest style refrigeration unit, not a front load upright for the best energy efficiency. All that is also explained in this article.