1. Debug with print (It's easiest way)

It's easy way to make a function that output variable to standard output and return the variable.

(defun dbg (t)  (progn  (print t) t ))

2. If you know of REPL

If you use 'break', you can see stack traces in REPL.

 (progn (break "hello debug dayo")
 ;; target code
 )

3. It's my recommendation.

If you use 'step', you can see the variable when the target function is called. So it's best for me.
You need to surround a target function with progn, but you don't need it if you use 'step' function.

 (step (something))