Betty Botter bought some butter, but she said the butter's bitter.

At a glance

B and T alternate with shifting vowels. In American English the T becomes a flap, which destabilises it further.

1
Say it once, slowly
Read the whole line aloud at half speed. Get every sound correct before you think about pace.
2
Break it into chunks
Split the line into two or three pieces. Repeat each piece on its own until it comes out clean every time.
3
Join the chunks back up
Say the full line again, still slowly. Accuracy first — speed is worth nothing if the words are wrong.
4
Start the timer
Hit start and push your pace. You need 15 clean repetitions in 60 seconds to pass medium.