Teaching my mother in law to use Signal:
Install. Accept permissions. Choose a PIN. I can send you messages. You can see your friends who already use Signal. Well done!
Matrix:
When you start you will get to choose from [matrix.org], swirly symbol "element matrix services" (the biggest and middle option, but not the one you want), or other.
Choose matrix.org (nothing else, or we will get very confused). Then click "sign up". Then enter a username and password (need a username which no-one has used before, you can't use your email address. Make sure to remember it).
Oh, but do put your email address in the next page, which is how people in practice are going to find you.
Bonus feature: How do you join a room? Click on it. How do you leave? Maybe long-press the room name, or swipe? Nope, you enter the room, click on the triple dots, settings, scroll down, leave room.
Bonus feature 2: Once you join a popular room, all users of that room are added to "Known users", making the list completely without value. How do I actually find people I know?