How to prepare ferrous Ammonium Sulphate?

Mohr's salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulfate and ammonium sulfate in water containing a little sulfuric acid, and then subjecting the resulting solution to crystallization. Ferrous ammonium sulfate forms light green crystals.
en.wikipedia.org/wiki/Ammonium_iron(II)_sulfate#Preparation