We provide a video that shows step-by-step instructions for how to measure diastasis recti. Here are a few troubleshooting tips to avoid the most common error I see, which is lifting the head too high when recording the measurement.
Always ensure you are lifting your head a bit from the floor to get the muscles to engage. Your true measurement will be just as the muscles begin to fire and grab the sides of your fingers. It's important not to lift too high because the higher you lift the head and/or shoulders, the muscles will automatically begin to come together and give you an erroneously narrow measurement. That said, if you are having trouble feeling the muscles at first, it is ok to first lift and lower a couple of times significantly higher to sense what it feels like when the muscles activate. For depth, it is necessary to press down with some firmness. We are measuring how far your fingers sink into the gap.
Another tip is to close your eyes while you measure so that you are not raising your head too high in an attempt to see your belly. Close your eyes and let the tactile feeling of the muscles grabbing the fingers guide you!