You can give the decayer any list of particles. It will look through the list, find the heaviest unstable particle and decay it. This involves going through all the known decays, from the highest branching fraction to the lowest, and replacing this particle with its daughters. Then the whole process starts again with the new list of particles. Subsequent decays are nested using lists, and once a final state has been reached that satisfies all the criteria, it is shown in a white box with the text aligned to the right. Once it has decayed all the particles into all the possible final states, it adds up all the branching fractions for each final state and displays them all, so that you can see how many times D+→ π+π+π+ π-π- takes place via all intermediate processes.
The decayer is a work in progress, so it may take longer than you think it would to reach stable particles. In fact, it frequently times out. After 29s it gives up, jumps out of all the nested lists and gives the (incomplete) output as it appears at that time.
To decay a list of particles you need to type in their Monte Carlo numbers into the first box.
To make the decayer useful to as many people as possible, the Monte Carlo numbering scheme used is the one set out by the PDG. You can see it [pdf]here.