How does "volunteering as hospitalero" work?

Depends on the albergue and the hospitalero.
If it´s a private organization or a business, you negotiate your own terms.
If it´s a municipal or church-run albergue that´s part of the Federation of Amigos of the Camino, you take a training course that fills you in on expectations, first aid, bookkeeping, etc., and provides you with a modicum of insurance coverage whilst serving in an associated refuge. The courses are offered periodically through Spain and via national pilgrimage groups in several other countries.

Or you can just offer to stay around and help out for a day or two or three, when you see there´s a need and the management seems willing. Great friendships have been born that way!
good luck!
Reb.

I think most albergues like you to work for at least two weeks. You can apply for a longer period though.

Some albergue websites have hospitalero request forms eg:
San Roque albergue in Corcubion - http://www.amigosdelcamino.com/ -
or even calendars where you can choose your fortnight eg:
L'Esprit du Chemin in St Jean Pied de Port - http://www.espritduchemin.org/

You can also offer your help directly to the refuge - a list of refuges can be found at
http://www.jacobeo.net - Click on Alojamiento for the different routes.

Write to Acacio da Paz - voluntarioscamino@ yahoo.es

Los Amigos del Camino de Santiago de La Rioja coordinates the care of pilgrims in the shelter of Logrono. You can contact them via email ruavieja@asantiago.org

Or in France:
Vézelay route
Amis et Pèlerins de Saint-Jacques de la Voie de Vézelay
http://www.amis-saint-jacques-de-compostelle.asso.fr
24 Rue Saint-Pierre - 89450 Vézelay Tél. : 0033 (0)3 86 32 38 11

Le Puy route
Conques Hospitalité Saint-Jacques http://www.amis-st-jacques.org/pages/hospitalite
Contact Pauline Dobon, Abbaye Sainte-Foy, F12320 Conques tel 0033 5 65 69 85 12
email acceuil-conques@mondaye.com

Good luck!