There are 2 major ways to get it done
1 - Using directly Tomtom gps device to create/add POI(s)
2 - Using tools to create POI category (.ov2 and its bmp files) then copy to the "map folder"
Item 2, for tools, there are several ways, one is creating using such "makeov2" file, the other way (I believe most of us using this) is using online POI Editor from various sites. NicolasG has provided one of them.
I, myself, find it a pain to create the custom POIs. Pain, here, means it takes so much of my time. Maybe because I've done the wrong way, the stupd way or whatever.
I have to manually enter EACH location address, find out location coordinator (by converting to Lat/Long then to Decimal), verify each of them (only in my area), enter info for each of custom POIs such give it a name, phone, .....
Creating .ov2 file isn't an issue since it's done by online.