To debug or execute the test cases line by line. Follow the below mentioned steps
1. Insert a break point (see the question to know more How to insert a break point in Selenium IDE? )from the location where you want to execute step by step
2. Run the test case
3. execution will be paused at the given break point
4. Click on the step (Blue) button to continue with the next statement
5. Click on Run button, to continue executing all the commands at a time.