Begin with classic Latin. Visit nationalarchive.gov for a free beginner's Latin class. There are 12 lessons that cover all aspects of Latin grammar such as noun declensions, verb conjugations, adjective and pronouns. Devote at least one week for each lesson.
Build a Latin vocabulary. Find works by Cicero and other classical authors at the library. Read and translate these works with a Latin dictionary. Make a list of vocabulary flashcards, especially the more difficult words that are giving you trouble.
Focus your attention a particular period of Medieval Latin. Medieval Latin extends over a thousand year period that includes numerous political, cultural and geographical differences. The practical approach is to become familiar with the general history and then focus on the particular writers of a certain region or time period.
Find bilingual editions of the authors you are interested in reading. It is often possible to find bilingual editions of the works of important Medieval authors. The edition will have the original Latin on one side of the page and the translation on the other. An additional advantage are footnotes that clarify obscure references and meanings.
Read the Latin text one paragraph at a time. Compare your reading with the translation. It is essential to always first read the Latin or you will not develop your reading ability. Avoid the temptation to read the translation first.