To help someone by phone, ask them to read out loud what is written on the screen. Then give one instruction at a time.
Ask the person to read out what they see.
Give one instruction, then wait until they say « done ».
Talk about shapes and colours: « the blue button at the bottom ».
Ask them what they see after each step. You should hear the new screen described.
If you are both lost: ask them to go back to the home screen, then start again. A video call also lets you show the screen using a second device.
Limitation: never install remote-control software at the request of a stranger. And never ask for a code received by text.
To picture it: it is like giving directions by phone to someone who is driving: you cannot see the road, so you ask where they are before each instruction.