When I asked professors to be my advisor I started my emails with: 'Distinguished prof. X,<br />
I am a student of ---, and i have followed your lessons/work. I would like to write a thesis about ...(subject of your thesis) which is a subject tthat falls within Your field of study. I respectufully ask your assistence as an advisor if you're available'...<br />
Hope i helped you! if you need help with your thesis just ask, I just handed mine !:)
Just straight up ask. Professors respect honesty. Tell him/her you respect them and their opinions, and you would like them to be your thesis adviser.