Speaking backwards is a cool party trick, but it actually involve a lot of practice and repetition. To start, it's best to have a piece of paper and a mirror. You will also need a recording device that allows you to play forwards and backwards.. The next tip is to write down how the words sound versus actually writing the letters backwards.
You need a voice recorder so you can practice the subtle way to sound out the words as you need to focus on breaking the words apart into their sounds. An example is MIRROR would be prounouced RORIM. You need to focus on the actual sounds that are made versus syllables for backward speak. But this is a useful trick or "code". Just be mindful that some words sound inappropriate or vulgar when said backwards in case you are practising in a public area. There are also computer programs and applications you can download to reverse play phrases and sentences or even see if your favourite songs contain "backmasking" a way to encrypt backward speech into the song.
There are certain tricks to look out for when learning the art of backwards speech. Sh or ch forewords isn't the same as backwards. For example: saying 'speech' backwards should actually say 'cheeps' and not 'hceeps'. It is very important to listen to how your pronounce the words backwards so you can plan the proper sounds to make the words sound appropriate. If your recording device has a loud playback this can be a funny icebreaker for a party! People will thing you're speaking a foreign dialect or some alien type language. Another "intergalactic" species is the loveable and quasi-backwards speaking Jedi, Yoda.
If you have always enjoyed codes or playing with speech, the below link at "About Entertainment" has an interesting article on Yoda-speak, even if it was just an invented language for the Star Wars movies.