The phonetics of the Spanish language make for a difficult transition when learning English, which is anything but phonetic.
English grammar has been taught in Spanish schools for over 15 years, however, as a subject rather than a language. The practical application is somewhat lacking.
Consider this when teaching English. What may seem obviously and simple to native English speakers could present a world of problems for a student of English as a second language.
Find everything you can about the common mistakes made by students and pronunciation difficulties. Make these the focus of your classes. –ED endings and the use of prepositions are aspects of grammar that need constant attention.
Evidence has shown that the more relaxed an individual, the easier it is for them to assimilate information. A friendly and comfortable environment can help create an open minded approach for both you and the student.
Never assume that you know everything, be willing to learn and accept your mistakes.

And most of all, Have fun!