It's a 2 part epoxy putty, you get 2 rolls of putty, cut off equal amounts off both and knead together and press on.
If it's an area that keeps falling off, make sure it's solid and doesn't flex. I'd also give the background a key of sorts... make it as rough as you can and push the filler in there as much as poss.
Milliput is great stuff for modellers, it not only fills, it can be lathed or you can make moulds with it and cast from them with the same. And if your better than me, you can scuplt awkward parts with it. The best bit is wet hands or a brush will wash it smooth and/or merge the filler with the background seemlessly without sanding.