Hi and welcome. You will find lots of good advice on this site. Have a look at Working in Spain and Moving to Spain for a start.
You first need to pick where you want to live in Spain, near the coast, inland, Spanish area, Brit area or whatever. You say you are only coming for 3 to 6 months. I think you would find it very difficult to get a job outside of bar or restaurant work for that amount of time. There are some English hairdressers serving the Brit expat community but I wouldn't think they would have many vacancies as they are usually one or two person concerns. You would obviously be looking to rent for that time so you need to check on availability and prices. The summer months are more expensive than the winter months (obviously) and the closer to the beaches, generally the more expensive accommodation charges. Look to pay at least €250 a week in the high season (which is now £250 a week). You mention your boyfriend has a trade but not what type. There are plenty or odd job men, carpenters, gardeners, painters and decorators all plying their trade near the Brit communities so that would be hard to get into. If you are outside these communities, a good command of the Spanish language is a must. To be honest, with the amount of red tape required in Spain to get legal work, sign on the social to pay your insurances etc. then 3 to 6 months is not a viable option. Bar or restaurant work is mostly casual but the pay is under the minimum UK rate even with the exchange rate at around 4 to 5 euros an hour. You'd have to work a lot of hours to pay the rent on a holiday home.
Anyway, best of luck with whatever you are doing. It may be best to visit the area you are thinking of for a week or two holiday first before committing yourself to a longer period.