Other people have said this, but not directly:

Start with the outputs.

Without some outputs, you have no way to know what's going on.

Beyond that, you have to adapt to what you have handy.

In my case, I've used a routine called "ping" that sends a bitstream out a pin, or "pong" that sends a byte in PWM  (WnnnWnWn would be 8Ah) because I have a nice four channel DSO to grab that with.

If I didn't have that, I'd use three pins, and drive an LCD, or something.

Back in the old days, we hung a pair of DACs on the Z-80 bus and connected each to the scope in X/Y mode, to get a display of where the processor was spending it's time.

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