Water pump removal/fitting procedure

Hi guys, I have a leaking water pump on my car and need some advice how to remove the belt so I can fit a new one (there is no PAS). Instead of buying a Haynes manual I would rather invest in the pump itself and ask someone who did this before.

I bought the pump and found a manual online that says "fit new O-ring" (unfortunately there was nothing about the belt removal procedure). There was no O-ring with the pump, should I go back to the shop? I don't mind the gasket missing, I will just use liquid gasket.

Thanks, Aaron.
does the new O ring come with the pump, or will I have to get it separately? There was only a water pump in the box (Dolz is the make, from Spain)