|The horse aren't actually on coins but I did them as well.|
It amazes me that some people who are much better painters than me don't bother to do this, but will go staight to flocking and thus leave a very obvious, unrealistic and ugly ridge at the edge of the cast figure base.
I have used cheap, Pound Shop, filler for levelling, but I fear it is rather powdery and brittle so I returned to using Tetrion, a robust filler with a proven record.
I apply the filler with a thin coffee stirrer, scraping it off around the rim of the coin and then prodding it towards the edge of the cast base of the figure. For this job I use the filler neat or with a tiny drop of water.