To conjugate a Strong Verb you need to know four pieces of information:
The infinitive: translated as "to xxxx". "To read" is the infinitive in the sentence "Alfred liked to read vernacular books." In Old English the infinitive will end with an.
The 3rd person singular in the past tense -- for example, "rang" in the sentence: "He rang the bell." [Many grammar books use the term preterite for past tense.]
The past tense plural -- for example, "sang" in the sentence "They sang the song" (it's the same for 1st, 2nd, and 3rd persons).
Note: for probably 80 percent or more of the Strong Verbs you'll be translating you won't need the past participle, but it's a good idea to learn it anyway, since it's the principle part from which Modern English forms of Old English strong verbs are drawn.
There are seven classes of strong verbs. You can use the following poem to help place a verb in its proper class:
The cat will bite the bird that will not fly
and spring upon the mouse when he comes by.
He gives no quarter and takes no guff.
A fool he holds him who falls for such stuff.
-- by Patrick W. Conner
Mnemonic Tip: Memorize the poem.
Taking the Modern English verbs from the poem in order gives us examples, in order, from the seven Old English Strong Verb classes:
Class I: bite = bitan Class II: fly = fleon Class III: spring = springan Class IV: come = cuman Class V: give = giefan Class VI: take = tacan Class VII: hold = healdan
Unfortunately, different grammar books use different conventions in numbering the verb classes. Some use Roman numerals as we do, but others use Arabic numerals. When in doubt, look at the table of abbreviations that usually appears either at the beginning of the book or the beginning of the book's glossary.
If you learn the principle parts of each of these verbs, you'll have the Strong Verb system.
Since strong verb classes are based upon a word's vowel or diphthong, you'll be able to match new words with the patterns you've memorized. For example, if you encounter the word "dreogan" ("to endure"), you'll notice that the diphthong "eo" is the same as the diphthong in "fleon." You'll then know that "dreogan," like "fleon," is a class 2 strong verb and follows that particular paradigm.