Each English letter on your keyboard is mapped to the Hebrew character with the closest sonority: d → ד, g → ג, r → ר, etc.
Some sounds can be written using several Hebrew letters (for instance, "a" could be either א or ע). In this case, just press the same letter again until you get the Hebrew character you want. For instance, if you want the letter "ע", you should press the "a" key twice.
Oh, and don't worry about final letters (ץ, ף, and so on). Just type the regular letter and it will be changed automatically into its final version if it is at the end of a word.