This is a copy of my Compostela. Even got my name in Latin on it. I, along with many other pilgrims that day, presented my stamped Pilgrims Passport or "Credential" to the office beside the cathedral in Santiago de Compostela. They check your credential, and when satisfied that you have genuinely made the trip, issue you with the "compostela"
Along your weary way, from your start, in my case Seville, up to your arrival at Santiago de Compostela, you carry your Credential and get it stamped every day along your route, to provide "proof" that you have indeed made the journey. You can get your Credential in the UK from The Confraternity of St James. Other countries have similar organisations. You will read on the web that it is possible to get a credential in Spain, but personally I would not rely on being able to get one on the spot, and certainly not immediately.
I found it a little amusing that people would choose to cheat in making the pilgrimage, and try to get a Compostela as a result of making the journey in the comfort of their car. But presumably a sufficiently large number have tried to dupe the cathedral authorities (and ultimately God, if they are believers), that "proof" is now required.
The first pages just set out who you are.
There are more inside pages than you could ever need for the stamps (or "sellos" as you ask for in Spain). Not that many people start at Sevilla, so it was interesting for me to discover that the cathedral in Seville is more concerned with extracting money from tourists than servicing pilgrims.
The santiago-today
web site suggests "You can get the pilgrims' credential in Sevilla
at the Cathedral, which is not always easy to do. Except for Saint Peter's in
Rome the Seville Cathedral is the largest, anywhere, and getting around it may
be daunting. The office that dispenses the credentials is located by one of
the six "main" doors and it has a tiny medieval-
type door which can be easily missed. Further, its hours of operation are pretty
much subject to Andalucian ways.... which means that maybe it will be open at
the posted hours, or maybe it will not. "
I tried asking at the recommended door, but the door-keeper was more concerned with getting money from tourists, than allowing me to enter for a "sello". In the end I gave up with the church, and got my first stamp from the Seville Tourist Office. Ultimately it really does not matter who gives you the sello, its really just sentiment that makes one want the first stamp from the cathedral
Along the way, I started by looking for Guardia Civil stations, or churches, but on the bike this proved difficult. For a start most places in Spain close for siesta from 2 pm to around 5 pm. Secondly most of your cycling time is in the countryside, away from towns, so the chances of passing an open Guardia Civil or Church is not high. At the end of a tiring day there seemed little pont in doing anything other than getting the hotel to stamp my credential.
And when I got to Santiago de Compostela, the pilgrims' office beside the cathedral were quite happy with my stamps, and the Compostela was handed to me without any misgivings on their part.