I mentioned somewhere on the other threads that I believe everybody has the ability to "play by ear" unless they are profoundly deaf, of course.
However, for various reasons, they have never developed this skill.
Perhaps, they haven't had much interest in music until now or have "forgotten" after years of formal musical education and relying on "the dots".

As others have suggested, the best way is to pick a tune or song you know "off by heart" and try to find the notes of the melody on your instrument. In all the various keys, if possible. It all depends what you play and arguments here about what is or what is not chromatic aren't particularly relevant or helpful here. Keep it simple.

There's a lot of controversy about learning scales but I would suggest it's a good idea to find out *for yourself* where the "doh, ray, me......etc" are...in each key. You don't even have to worry too much about what key is which as long as you can learn to find your way about the instrument.

Now the above is assuming that you haven't played a particular instrument before and, possibly, have no previous knowledge of music.

I can imagine there's likely to be more difficuly if you already play music from "the dots" as you have to "unlearn" a few things or, at least, put them to the back of your mind.