Not sure if I understand correctly. You want to stop when entering the sub and then step forward for debugging?
If so, make a click with your mouse left beside the first line of code within the sub beside the line number (a red dot appears left from the number), then switch to debug mode in the controls. Run the app in debug mode. If entering the sub, the program will stop exactly at the line with the red dot without executing the code of the line. Then you can step through your program line by line and control variables at runtime.