This is the job Basecamp is best at. Building a training block on a phone is possible; building one with a keyboard, on a screen that shows the whole program at once, is a different experience entirely.
Adding one
Select the client, then Add workout. It takes a title, an order and notes. The workout appears on the client's phone immediately. There is no draft state, so an empty workout is a visible empty workout.
Order is a number here
Unlike the phone, Basecamp gives you an explicit Order field: lower numbers come first. It has to be a whole number of 1 or more, and you are told if it is not.
Typing a number is much faster than dragging when you are building six workouts in a sitting. A useful habit is to number in tens (10, 20, 30) so you can drop something in between later without renumbering everything.
There is still a drag-and-drop Workout order screen when you would rather move things by hand.
Two dates worth reading
Each workout in the sidebar shows when it was updated and when it was last viewed by the client. The second one is the interesting number and it does not exist on the phone.
A workout you wrote three weeks ago that has never been viewed tells you something a completion count does not. Either the client is not getting that far down their program, or they did not know it was there. Both are worth a message, and neither is visible anywhere else.
Naming them for the client
Your client reads the title on a phone in a gym. "Lower body A" is your shorthand; "Legs: heavy" or "Monday workout" is theirs. Put block names in the folder rather than repeating them across every workout title.
Editing and deleting
Edits are live the moment you save, so avoid rebuilding a workout somebody is halfway through. Basecamp will tell you if they are in the app right now.
Deleting asks you to confirm and cannot be undone. If something is finished rather than wrong, move it to a folder. You keep the record of what the client actually did.