I take it from your previous thread that you will need work, or do you have a business that could be run from Spain? Everybody is always dead set on moving but don't be fooled by holidays in the sun. Are you planning to be self employed?

Before anything else you will need to arrange health insurance for yourself and any family members that will be living with you.
you will need to prove you have enough income so not to become a burden on the state.
you will need to arrange a Spanish bank account.

You will also need to prove you have money in the bank - £10k + in some cases i've heard

As it stands pre Brexit a bank balance is NOT a legal requirement, what you do need to show is sufficient resources so as to not become a burden on the state and normally that is demonstrated by the three months worth of bank statements typically demanded when applying for residencia showing regular income. A bank balance does not necessarily satisfy that as not only can it be withdrawn and spent in a heartbeat but it also offers the opportunity for fraud by those so inclined who might borrow money expressly for the purpose of obtaining residencia only to have to give it back immediately after!

​​​​​That said we all know that what's actually demanded can vary from region to region (and even by individual official) so the least unreliable guide will be first hand reports from others who have been through the process in the specific area and office in which you yourself will be applying.

It seems pretty certain that post Brexit the level of resources demanded will increase significantly to match those currently demanded of non EU immigrants.

Here in Valencia a friend of ours had to not only prove income but also had to have €12k in their Spanish bank. The €12K had to be shown that it came from their (home country) bank account and they had to provide statements to prove that the money had been in there for a while and not just transferred by a friend or family member a few days previous. The officers also checked their last 3 month Spanish statements to see if they were regularly paying rent, utility bills, food shopping etc... This is what was required here a few months ago. However this can change as mentioned above and the requirements differ from region to region. If you state where you are looking to move to maybe someone can provide you with more specific information.

First time I have heard them checking like this. Surely your friends had to make a number of visits as the paper work required plus competent translations of British statements is not the normal documents required?

They used a company to help them and got it done in one visit. The company told them what was required as they do it regularly. If they had not used the company then yes they would have had to do numerous visits with a translator as their Spanish was not very good.