I’m totally with Bakermam…I leave a 1cm over lap, pull the ribbon till it’s snug the a dot of royal icing. The beauty of RI over piping gel, edible glue or buttercream is that you don’t have an oily/wet mark showing thru. It dries quickly hold well and won’t make the colour of the ribbon run. I wouldn’t wet the ribbon for fear of it drying patchy (especially silk ribbon) and strong colours such as red will bleed onto the cake.
I also agree with SaraBeth regarding buttercream. If you let it crust first, not only will it make it easier to apply the ribbon without damaging the finish, it stops the oily ‘bleed through’ from the buttercream ruining your ribbon.
Hope that helps :) x