A client you have worked with for a year has a lot of workouts. Folders group them into something readable, and Basecamp gives you more control over them than the phone does.
Creating one
New folder, then four things: a name, a description, a color and an icon. Only the name is required. Leave the other three blank and the folder still works, and your client sees the name on its own.
The description is not for you
Basecamp is explicit: your client sees it under the folder name in their app. So write it to them, not about them.
"Six weeks building strength before we move to more volume, three workouts a week" is worth more than "Block 2". It is one of the few places you can tell a client what a phase is for, and it is easy to leave empty.
Colors and icons
Eight colors (slate, amber, orange, green, teal, blue, indigo and purple) and a set of icons including a stopwatch, scales, calendar, lightning, sunrise and moon.
These are only useful if you are consistent. Pick a scheme and apply it across every client (one color for strength blocks, one for conditioning, one for rehab) and your whole roster becomes scannable. Choose at random and they are decoration.
How to divide them
By time beats by body part. "Jan to Mar" tells a client where they are and stops growing; "Legs" duplicates what the workout titles already say and expands forever.
An empty folder tells your client nothing is filed there yet, so build it when you are ready to fill it.
Folders are labels over workouts that already exist. A workout is either filed in one or unfiled, and filing it changes where it appears rather than what is in it.
Deleting one is safe
The workouts and exercises inside are kept. Basecamp says so on the delete control and again in the confirmation: the contents stay, they simply go back to being unfiled.
That makes folders genuinely low-risk to reorganize. If a structure is not working, take it apart. You are moving labels, not training history.
Once the structure is there, Building a workout in Basecamp covers filling it, and Copying workouts and exercises covers reusing a block you have already written.