Select a word with at least four letters. The longer the word, the more letters you have to work with, but the harder it will be to find a single-word anagram.
Break the word down into individual letters. You can do this mentally or, if it helps you, visually, by writing letters on individual slips of paper and rearranging them.
Experiment with different letter combinations until you can assemble a new word. Keep experimenting until you begin to see the new, anagramic word emerge.
Consult a letter combination chart to help you. Such charts list common letter combinations (blends, digraphs, trigraphs, etc.). For example, "spr,"th," and "na" are typical combinations that will help you construct your word.
Plug the word into an online anagram converter if you want to find anagrams automatically. There are numerous such anagram tools available online for free.