Consult with your client to ensure that they are capable of sitting for the tattoo. You also need to explain it will be expensive and the tattoo will take some time. As long as you are on the same page, you are ready to take your deposit and book out your appointments. Consider the style of the piece and size of the arm you are working on. Make sure that you are not only thrilled with the project, but capable of executing. It will be time consuming, huge and a permanent billboard of your skill.
Bring your best skills, you will be building a relationship with this person. You may even become friendly when you have a great positive attitude from the start. Neither of you should take the project on if you are incapable of handling long term commitments. There is more to a sleeve tattoo than being a great tattoo artist, you need to be able to decipher a person's vision. Without listening and communication skills, your sleeves will fall flat with your client. They will photograph well and look great, but they won't complete your client's vision. With only two arms, committing to a sleeve means you are responsible for a lot of skin real estate.