If you see
"monter les blancs en neige ferme" it means they must be very thick and not liquid at all.
How can I do this ?
Use an electrical whisk. It takes about 4 minutes depending on how many "blancs" you need. Do not "monter en neige" more than 6 whites at a time. It's easier to do this if you take cold eggs.
Then, if you use these "blancs en neige" in a cake, mix them DELICATELY, use a spatula.