Now this has been bugging me for a while. How do I know when to make a noun feminine, or in other words, add the -in- suffix? Okay to start simply:

Mi estas knabo. Mi estos viro. | Mi estas knabino. Mi estos virino. (Fine so far. Boy/ man and girl/ woman are states of being.)

I read in the course La Zagreba Metodo in Amiko Marko:

"Lia patro estas laboristo, li laboras en hotelo. La patrino instruas. Ŝi estas instruistino." (Male form and female form of the profession.)

Though when Ana Pana is talking in her course she says in the sixth lesson:

" Eĉ malpli facile scii kio mi ŝatus fariĝi! Ĉu programisto, ĉu instruisto, ĉu aspektigisto, ĉu flegisto?"

Ana Pana is a girl (I am assuming here) so shouldn't she put the -in- suffix here? Or is it that when we are talking about becoming something, or being something, in relation to another thing you use the male noun form? Like, I might become this in general.

Like "Ĉu programisto.." but when she is a progammer she will say: "Saluton, mi estas programistino...."

In French you can sign off an email: Ana, Étudiant. Usinhg the male form to state membership of a group. But: Je m'appelle Ana, je suis étudiante. Using the female form to tell more about yourself.

Sorry, I know what I want to say but I just can't find the words to articulate myself fully.

Can anyone offer some insight? Caeireann.

You only have to add -ino if the word is male by default, but refers to a female. That includes family words like patro, frato, avo, etc, and titles like sinjoro, and terms of nobility like reĝo, barono, senjoro, etc. There are a few others as well, like knabo and fraŭlo. These types of words require -ino if its referring to a female.

Such words are the exception though. 99% of nouns are neither male nor female by default, including words that end in -isto (and other profession words like aktoro), words with a participle ending (e.g. parolanto, skribinto), and so on. These don't need -ino -- but you can still add -ino to them if you want to. So "instruistino" isn't wrong, but using "instruisto" referring to a woman isn't wrong either. The latter is more common I think.

Bottom line, if what someone writes or says makes sense, then it's good enough.

PMEG goes into this subject in more detail.

Thanks a lot I understand how it works now Caeireann

I think that's a personal decision and should be left to noun itself.