The Bum Dit-ty

This is the core rhythmic technique of clawhammer playing. It involves hitting string 3, 2, or 1 (string 4 ias the nearest to your head) with the nail of the index finger on the downstroke usually for one beat of the four beat bar, followed by another similar downstroke or brush of combination of strings 3,2 and 1 on the first quaver (eighth note) of the next beat. As you play this second downstroke the fleshy underside of your thumb comes to rest on string 4 (our drone). On the second quaver of this second beat there thumb then pops / rolls off the string to play the drone note, the open 4th string. It´s easier to do than describe in words ! The thumb note should ideally be quieter than the downstrokes, so the melody notes stand out in front of the drone.
Here are some useful exercises:

F denotes the index finger playing a downstroke on the third string only, T the thumb top / roll. Play bar 1 over and over by itself alongside your metronome (real or on your phone). Then work on bar 2 so you control the second downstroke to brush two strings. Repeat it by itself. Then work on bar 3 where the second downstroke brushes three strings of a C chord. Bar 4 is a repeat of bar 1. When you are happy with controlling the scope of the second and fourth downstrokes play the exercise as a four bar unit. Start slow and build speed in 5 bpm steps until it is burned into muscle memory. Try to practice this every day until it is steady.
You will notice in the videos that I play just between the sound hole and the neck join to get a sweet tone. I also tend to hit the top of the uke with fingers three and four if the picking hand to add a percussive tap. In my playing this sounds like a dancer flatfooting in the background. It also helps me keep track of where my hand is so I can really dig in when I need to.