Tips for playing in thumb position (on cello)?

In the first week, find the harmonics with the thumb and then press the string slowly and carefully, to see if you can press it down and keep the tune, just for a minute or so at a time. Try it on different notes if you like. Keep your hand in a fist so your thumb and forefinger looks like a snail's shell. This will help too apply pressure without much effort.

Hope this helped!