What is the first step for fitting a testing plug to the wall of a drain?

The first step for fitting a testing plug to the wall of a drain is to clean the pipe and line up the plug and pipe. This step is crucial because a clean surface ensures a proper seal when the testing plug is fitted. Any debris, grease, or build-up on the pipe can prevent the plug from sealing effectively, which can lead to leaks during testing. Proper alignment between the plug and the pipe also helps facilitate a secure installation, ensuring that the testing process can proceed safely and accurately.

The other options might pertain to relevant procedures in specific contexts, such as sealing connections or checking environmental conditions, but they do not represent the initial preparatory step necessary for the effective installation of a testing plug. Proper cleaning and alignment are fundamental practices in drainlaying to ensure the integrity of the drainage system during testing.