It is all about the syllable with the most force. I'll assume you know how to separate the syllables, but if you don't feel free to message me. Syllable numbers: 1-2-3-4 Agudas: The last syllable has the most force. You add the "atilde" if the syllable finishes in N,S, or a vocal. Example: avión, mamá, señor. Llanas: The 3rd syllable has the most force. You add the "Atilde" if the syllable doesn't end in n,a, or a vocal. Example: lapiz, mesa. Esdrujúlas: The 2nd syllable has the most force. You always add the "atilde". Example: química, analítica. Sobreesdrújulas: The 1st syllable has the most force. You always add the "atilde". To really learn them, I recommend you do atilde exercises and have someone who really knows Spanish correct them to you. :) Hope it helps!

http://www.uamenlinea.uam.mx/materiales/lengua/ortografia-20%20jul/Palaagud.htm This site is in Spanish. If you want help with it, let me know.

I was taught the "NOSE" rule, being that... If a syllable is otherwise accented, the stress of a word falls on the final syllable unless the word ends with either N, S or a vowel, in which case the stress falls on the penultimate syllable.