Wash the rhubarb, trim the stalks and discard the base and any leaves. Cut the stalks into 3cm lengths. Put in a large jar with the sugar. Shake everything around, put the lid on and leave overnight. The sugar will draw the juice out of the rhubarb.
After 24 hrs, add the gin, seal and shake everything around. Leave for about 4 weeks before drinking. You can strain the liquor off through a muslin-lined sieve and transfer to a bottle, but I often just leave the rhubarb and booze in the jar and ladle it into drinks that way. Over time the rhubarb and the gin go a much paler colour – this doesn’t look as dramatic. The upside is you that have to get through it fairly quickly!