Turning the phone on (using power button for example) while holding the Vol+ button is enough to put the phone into flashing mode. The problem is that there’s absolutely no indication that it did turn on - everything remains as black and dark as when it’s off, and the only way to know that you succeeded is to connect it via USB and to check whether it shows up in flashing mode on the bus.
…which is why the instructions tell you to do things in a way that gives you immediate feedback; however, when you know how it all actually works it turns out you don’t have to challenge your dexterity at all 