Here is an example how to make one for e.g. Gnome on a Core install. (or on a KDE install)
all sets are stored in /etc/entropy/packages/sets , so in my example I'll use what will be our stock Gnome set for Core:
create an empty file:
and put in these atoms:
Save the file and do this:
equo install @gnome
The example set will actually be available in Entropy by default so there will be no need for creating this one, but I can imagine that you install Core and only want to install apache, PHP, MySQL and perhaps some additional packages. For such thing I encourage you to write a set and keep it stored somewhere safe in case you ever do a fresh install again.