To earn a Deal with the Devil, you have to take zero Red Heart damage in any room after Basement/Cellar 1. This will give a 100% DwtD chance on that floor.
There are other ways for a DwtD to appear, as shown here;
https://bindingofisaacrebirth.gamepedia.com/Devil_RoomDevil Deals require the trade of hearts to acquire an item. If you do not take a deal, the next DwtD may actually be a Deal with the Angel, and will make this trophy harder to get in that run.
If you are having difficulty amassing enough hearts to take the deals, remember that 3 red heart deals can be taken by only having 1 red heart.
It is ideal to attempt this trophy as early as possible to unlock Azazel, as he is arguably the "Easy Mode" character, with the ability to fly, fast speed and a short Brimstone laser as his spawn tears.