You can find videos on Youtube showing how to decant. I've done it, and yes it can be messy if you dont do it right. I followed the technique on the video...somewhat. The containers I used worked fine for the Testors paint; however, Tamiya sprays, being lacquer based, melted the bottom out of the container I used. It worked fine initially, but a few days later I went to use the paint and the container had become fused to the work surface.
I have since bought some better glass containers with lids to decant into.
One of the main things to remember is that you MUST not seal the container of paint that has been decanted for several hours. It has to off-gas the propellant in the paint otherwise, even the heat in your hands can cause a massive boil effect and you will have paint everywhere! To be on the safe side, if you decide to try this, I recommend doing it outside!