How to get glacé icing right on a cake?

You have three options:

1. Prepare for drippage. If you're doing a lot of small items in particular, you set up a wire rack over a sheet pan, and just let it drip down. For the small items, you can dip into the glaze, then invert and place on the rack, like you would glaze a donut.

2. Add a barrier. Make a thicker icing that won't flow, and pipe right at the edge to give it a little bit of containment, then flood the top with the thinner icing. You'll need the cake nearly level for this to work well.

3. Pipe it on. You need an icing that's just thinned to the point where it will slump and flow back together, but not so thin it'll flow off. Use a piping bag (or plastic bag with the corner cut off) to make lines close enough together so the icing forms a solid covering. You may want to start at the outer edge and make a ring like #2, then work inward. This can work for more uneven cakes, as it doesn't need to be perfectly level, but it's more effort to get the consistency right.

Exact technique and consistency is going to depend on how thick (depth) you want the icing. It's always easier to thin it (doesn't take much added liquid) than to stiffen it back up.

I addition to Joe's three options, you could try a couple other things

1. Making several thin layers - a thin layer won't drip nearly so much, as it doesn't have the extra icing to spare to run over the edge. A thicker layer can spread as it flattens and slumps. A thinner layer can also dry faster, and once dry will help keep the next layers firm as some of the moisture that made it runny will get used up dampening the lower layers instead of keeping the icing flowing.

2. Leaving a gap at the edge. The icing will flow and slump a bit as it settles, so giving it room to do so should prevent it overflowing the edge. You could work upwards from an eighth, to a quarter or even a half inch gap around the edge until your icing no longer overflows the sides - you can go back afterwards and pipe some drier icing around the edge, and/or use a drop of water to smooth the edge outwards until it looks right.

3. Dusting the edges of the cake, or even the whole top of the cake, with icing sugar before icing. The loose sugar should absorb moisture from the icing fairly well, effectively thickening and drying the icing after it has been spread. This should help slow down the flow of the icing before it runs off the edge. You could also try dusting icing sugar on after the icing is spread, though this will likely affect the icing's appearance and texture (potentially fixable by smoothing down with a drop of water, but still an extra step).

Also, it may help to let the icing sit for a bit before use. Textures can change as ingredients hydrate, a mixture can become thicker or thinner as it sits. I would expect a sugar mix to become just a bit thinner, if even a minute fraction of the sugar will move from mixed paste to dissolved in the extra time. Waiting should eliminate the possibility that the icing thins in situ, causing the drips. It should also make it a bit less work to mix, as the sugar is already wet.