You need to provide some more information.
Firstly, are you UK citizens?
Secondly, no-one really knows what will happen in the future, ie, next year.
Thirdly, how old are you? You say you are retired but people can retire early nowadays.
Now, assuming you are from the UK and you are going to do it this year then the rules have been posted on here many times.
You will need an NIE which is a foreigners' identification number. Everyone needs one of these (Spanish have a NIF which is the virtually the same thing). You can get this from a National Police Station or foreigners' office in Spain. You can also apply for one in UK at a Spanish consulate in person.
Now, if you intend to move to Spain this year (until the end of the transition period in December) then the rules stay the same. You have to prove an address in Spaiin which is either by the deeds to a property you buy or by showing a long term rental contract. So far so good.
You have to show you have medical care either by private means or state provided. If you are over retirement age in UK you can transfer your healthcare to Spain by the means of an S1 which is obtained from the Overseas Health at Newcastle but I think (maybe you need to check) you need a Spanish address for that.
You also need to show you have the financial means to support yourselves. Depending on which region you are moving to, some National Police offices seem to have different rules. At the moment, for UK citizens, you need to show an income of around €800 per month per person although some regions simply say 800 euro a couple if you own your own property with no mortgage. Here, in the Valencia region, you also need to show at least 3 months of payments into a Spanish bank but other regions don't seem to enforce this. You need to check the amount required in the region you plan to move to.
Once you have all this in place, you complete an EX18 form to apply for residency and you have to do this within 3 months of arrival. You then go to a Nationa Police station or foreigners' office with all the documentation (plus proof you have paid the quite small fee) and hopefully get on the register of EU foreign nationals residing in Spain.
This may all change from next year but no-one yet knows what the rules will be. If UK citizens are treated like non-EU citizens then the amount of income to be shown could dramatically increase to about €32,000 a year per couple plus you may not be able to have your healthcare transferred from UK (all to be negotiated).
UK is bringing in the rulest that a minimum income of £25,000 will be required from EU citizens so there is no reaon the EU won't copy that.
Probably a few things I've missed but it is important that you do it this year to ensure you maintain some of the rights UK citizens enjoy at the moment.
Caveat: Until talks and agreements have been made, nobody has a clue on what will happen except that current UK citizens with legal residency in Spain have been told they will keep their rights after December with regards to income, healthcare, pensions and so on.