Every client has their own list. New to-do takes a title and optional notes. Drag the list into the order you want. The top is what you do next.
The list of everything
One view shows every open to-do across all clients, grouped by client. Open it when you are deciding what today looks like.
Ticking things off
A done to-do stays on the client's own list for a week, so you can see what you did. The list of everything shows only what is left.
Timing a to-do
A to-do can be handed straight to the timer. The clock starts against the right client, with the to-do's title filled in as the description. The hour you log says what it was for without you typing it twice.