It will depend on the type of cake. If it is a fondant cake you can stick it with water or edible glue. But if it is about sticking fondant on buttercream, the matter is more complicated since we cannot use glue or water.
If it is a figure, simply insert the skewer into the cake and that’s it. If it is in a side, you can also help yourself with a toothpick or skewer, depending on the size or let the cream soften a little to stick it. But be careful, if the cream is very soft or if it loses a lot of heat, fondant will be soft and it may fade out the colours and stain the buttercream.