Obtain finely powdered iron oxide (rust), aluminum, and a thin strip of magnesium.
Mix them together in a 8:3 ratio (iron oxide: aluminum) in a ratio by weight. [Note that the since aluminum is so light, it will appear that it is about a 50-50 mix by volume].
Pour the mixture into a strong container, such as a cast iron container. Note that if you ignite the thermite, it will melt the cast iron though.
[select your site carefully]. Make sure there is nothing that can burn within four meter radius of the reaction.
Insert and light the magnesium strip and it will burn out in a few seconds.