Something easy to do: GYM.

Stand erected on your two feet and then rise on your toes.
Go up, go down.
On your toes again
Repeat it
The first time you will probably do 20 before having pain. Then in a week you will do 100.
You can then do it in the morning and before going to sleep. Doing it 100 times take you arround 2 minutes.
By adding some muscle to your legs you will stand high heels much easily.

Carla Mel

PS Stiletto have a double dificulty not only you have to rise on your toes, but you have little help to keep equilibrium. If you want to train try first platform till you are confortable with the "form" of the feet and then train the stilettos.