While I completely agree with Chris, I'd like to make a few suggestions concerning the 'how'.
I'd suggest first writing / drawing a schematic of the plot, in order to get away from the written text. This means that no sentences can be taken out, just ideas and topics.
Then, keep away from the play for some time and will yourself to forget it as much as possible.
Finally, look at the schematic (which is basically the skeleton of the story) and start writing it in prose, making all the necessary changes (including to the plot, if need be) as they become necessary.
This approach means that things can become quite different from the original play, especially where it comes to dialogue, since you won't be going back to check it and keep it word by word. However it allows a great deal of freedom to reinvent the tale.
If the idea is to keep as close to the original as possible (which means respecting dialogues, events, etc) the first thing I would do would be to choose one character (or a few) and place myself in their shoes. The event, the dialogues, everything, will be seen through their eyes and ears.
That means that there may be scenes that are not in the book because the character(s) didn't witness them, which will require new scenes in order for the information to be transmitted in a new way.
It also means getting inside the characters more deeply than it was probably required for the play.
This approach is much more difficult and likely to require several drafts until it loses any touch to its theatrical counterpart while respecting events and dialogues to the utmost.
Dialogues, however, will be what will require the greatest change: in a play you need to have characters state their thoughts. These dialogues aren't necessary in a novel as the reader has access to their inner thoughts in the narration. It even means that some scenes can be scrapped completely, or that some situations can develop differently. That is the trickiest part and why the first approach I mentioned is often the best one.