Learn French: L'uniforme n'est ni confortable ni formel. - The uniform is neither comfortable nor formal.

The structure ne... ni... ni... means neither... nor.... Because ni already carries a negative meaning, it completely replaces pas. Saying n'est pas ni... would be incorrect.

How do I know where to place ne and ni?

The ne (shortened to n' here) goes immediately before the conjugated verb, just like in a standard negative sentence. The word ni goes right in front of the specific things you are ruling out, which in this case are the adjectives confortable and formel.

Can I list more than two things using this structure?

Yes, you can! If you want to say the uniform is neither comfortable, nor formal, nor cheap, you just add another ni before the next item: n'est ni confortable, ni formel, ni bon marché.

Why is formel spelled with a single L here?

The adjective describes l'uniforme, which is a masculine noun. If it were describing a feminine noun like la tenue (the outfit), you would use the feminine spelling formelle.