Choose between American and British English. The rules for American and British English differ substantially, though those differences do not interfere with the ability to understand and communicate with each other. Keep these differences in mind when choosing a textbook or an instructor, as discussed in steps two and three.
Get a reputable grammar text. A good text will cover subject-verb agreement, nouns, prepositions, adjectives and other parts of speech as well as sentence structure. The text should clearly present the rules and give the most notable exceptions to the rules.
Study under teachers who are trained in teaching English as a second (or foreign) language and who are native speakers. Do not enroll in courses given by untrained native speakers. Teaching English to a speaker of another language requires cognitive learning strategies that only teachers trained in the discipline can properly carry out.
Avoid jargon and slang. Attempts to learn proper English by watching popular television shows, movies or by chatting with people who are born in America, Canada, Australia, or in other English-speaking countries, but who have no foreign language teaching background, can defeat your goal to learn correct English grammar.
Read the classics. Find English or American literature classics to read English in its highest form. Study the words and structures to get a good feel for the written language.