Gather the material as lister above, get a hot stove, a beaker or a cooking pot and a scale

put 7 liter of water to 100°C (until it start boilling) and while doing so add all the flour to another recipient where you will mix it with the liter of water left until you have an homogenous and sticky mixture. Once it's done you can add the flour mix to the boilling water and keep stirring that it do not precipitate for 15 minutes. Once done you can jump to the next step

now you will add the Dye (Iron Oxyde),  the Iron sulfate and the copper sulfate to the mix while you keep stirring for 15 more minutes

Now you should add the linseed oil and stir for 15 more minute (I promise no more stirring after that)

now you can add the soap and let the mixture cool down. you should have a nice red paint that is slightly viscous and smell like rust and linoil. If you feel like your paint is not liquid enough add a bit of water and mix. You're paint is ready.