Spain provides an Spain Schengen Visa UK application form online and you can also book your appointment online. You can find more information on the Consulate of Spain in London.
Each country has its own requirements in addition to the supporting documents listed by the European Parliament and Council on the EU Regulation dated 2009. The Spanish Immigration Officer will ask you, for example, to provide proof of your financial ability to handle your trip (€65 per day).
In any way, you must lodge the application at the Spain Consulate/Embassy/Visa Application Center where you will have to spend the longest period if you have decided to visit other countries in Europe.
You have a minimum of 15 days to apply before your intended trip and you can’t ask for a Schengen Visa more than 6 months before you travel to Spain.
Be aware that if you are a family member of European or EEA (European Economic Area), you might benefit from an accelerated visa procedure free of charge in specific cases: the EU/EEA citizen is travelling or living in a country member of the Schengen area other than the one he/she is a citizen or you are planning to meet him/her or you are already accompanying him/her in Spain.