Hi Rasa. My webpage https://www.tenerife-information-centre.com/weddings-in-tenerife.html pretty much addresses all your questions, but the most important thing to stress is that only Tenerife residents are allowed to marry on the island. Non-residents must marry elsewhere, then come to Tenerife for a blessing ceremony.
Regarding what documentation is necessary, I've listed six Tenerife wedding coordinators on that same page, who would be happy to arrange all that for your clients.