Put an apostrophe after the stressed syllable.

for mal ly

I have no idea how I'm supposed to know where the stress is

The place where the apostrophe should be put is form 'ally.

Explanation:

All English words have syllables. An easy way to understand a syllable is, when you say words aloud clap your hands. One clap words have one syllable and they are not stressed out.

For example: Words like walk, talk, bought, bat e t c are said easily and have one clap.

But words like happy, angry, pretty, ugly e t c, when they are spoken aloud, require two claps, as these words are stressed and broken into two syllables.

Similarly for the word formally, it requires two claps when you speak the word out. This shows that there are two syllables in the word formally.