The amount of dough (batter) depends on the size of the waffle maker and your needs. The dough (batter) will expand a little after baking, so do not make it as large as the pan of the waffle maker, in case it might spill out.
To test whether the cake has been cooked, using bamboo sticks / chopsticks to stab into the cake center and make sure it is not sticky
Any high temperature resistance molds can replace for baking cup.
It can be place into the microwave; oven or double boiling to melt down the butter.
Any kinds of the butter which available in market that can be good for it. (Recommended the unsalted butter)
No need to worry about the temperature of egg, but do recommended beating the eggs before use it
You can search the recipe of some small snacks and cakes on the internet. Follow the steps for cake or dough making. After a while, it will be natural to you.