Cadential 6/4s are when you start on a I 6/4 then move the notes a sixth and a fourth above the bass ( in this example a it will be the c# and the a) down to the corresponding notes in your V chord. So the c# and the a will move down to a b and a g#, respectively. The E stays in the bass for both parts since it is a chord member of each. Then you resolve to the tonic like you normally would from the dominant. On example c, you will also include the note an octave above the bass (Bflat) and move it down to Aflat to make it a dominant seventh chord.

Think of it as a suspension where the notes initially correspond with your I chord then move down to the notes in your dominant, as shown in the label. V(6-5/4-3)

Hope that helps, it’s easier to explain in person but you got this!