Formal:
* Asseyez-vous et lisez. (This is the most literal translation and is the most formal option.)
Informal:
* Assieds-toi et lis. (This is a more casual way of saying it, and you can use it with friends or family.)
* Installe-toi et lis. (This means "get comfortable and read" and is a good option if you want to emphasize the relaxing aspect of reading.)
You can also use a more specific verb depending on the situation, for example:
* Installez-vous dans le fauteuil et lisez. (Sit down in the armchair and read.)
* Prenez un livre et lisez. (Take a book and read.)
* Installez-vous confortablement et lisez. (Get comfortable and read.)
The best way to choose the right phrase depends on your relationship with the person you are speaking to and the situation.