Since the financial crisis of 2008, banks have been reluctant to lend to non-residents in Spain. Gradually, they have started lending again and mortgages are now easier to obtain and issued more frequently.
Spanish and International banks issue the mortgages either directly or through mortgage brokers, which will scan the market for you, so you may obtain the best conditions. In general, lenders do not finance more than 70% of the property value (as determined by the banks' own valuation, which typically is lower then the market value) to non-residents. Another requirement, generally, is that you have at least 30% of your net disposable income, after taxes and debt obligations, free to service the new loan.
Unless you already have an established relationship with a bank that finances real estate in Spain, we would recommend that you use an independent mortgage broker.
Not only will they find the best interest rates and conditions you require, they also help to overcome the language and bureaucracy barriers.
If you are determined to acquire a property (partly) through a mortgage, it is strongly recommended that you pre-qualify yourself with a bank of choice (directly or through the mortgage broker).
Some of the documentation that banks and mortgage brokers may request you to submit are two years of tax returns, details of existing loans, bank account and credit card statements, proof of income, total assets and liabilities (confirmed by an accountant), personal documents and an application form.