Most metal moulds are stainless steel as it spalls less while at forming temperatures. The techniques here can be applied to any metal, although spalling will be a common occurrence on any metal other than high grade stainless steel.
On many single curve moulds, such as a partial cylinder, you can just lay fibre paper over the form and place the glass on top of that.
Metal moulds that have more complex shapes require a separator that will conform to those shapes. Applying liquid kiln wash requires you to heat the steel to somewhere between 150 and 200C before applying the kiln wash. Any hotter and the kiln wash will boil off on contact, leaving an uneven coating.
The kiln wash can be applied with a brush or by spraying. Spraying gives a smoother less streaky application. After giving the mould the first coat, return the mould to the kiln and re-heat the mould. Repeat this until you have covered the whole mould with a thin layer of separator. Be careful and avoid applying too much kiln wash at once, as that will cause the separator to run and reveal bare spots on the mould, causing you to need to clean and begin again.