From Wikipedia:

April Fools' Day or April Fool's Day (sometimes called All Fools' Day) is an annual celebration commemorated on April 1 by playing practical jokes and spreading hoaxes. The jokes and their victims are called April fools.

As the name implies, traditionally, April Fools' jokes over here are made to make the other person look a bit foolish: having them check that their shoelaces aren't untied, or having them believe some kind of implausible information. Usually, we stick with small stuff like telling people 'your shoelaces are untied', then letting them know how foolish they were for checking by saying 'April Fools!'.

But, sometimes, the joke goes too far. I've been in trouble in the past with people reacting badly to my jokes, but I've also been the party that 'can't take a joke' and experienced April Fools' jokes that I absolutely didn't find funny or appropriate as they made me more of a fool than I'd prefer to look, or actually resulted in (unintended) physical pain. Wikipedia also sort of points this out by referring to April Fools' jokes as 'harmless pranks':

Aside from April Fools' Day, the custom of setting aside a day for the playing of harmless pranks upon one's neighbour has historically been relatively common in the world

So, in order to make April Fools' day fun for both the joker and the fool:

What is suggested to keep in mind, to make proper April Fools' jokes that are both harmless towards the other person and at the same time make them look a bit foolish by having them believe or fall for a prank?