The simplest way is simply to have the butts lined up underneath where the water comes off the roof – if it’s an allotment, you don’t actually need a gutter – it can simply just fall straight into the butts.
With a gutter in place, you need someone handy to attach a side arm to the vertical downpipe which siphons off water through a tube into one butt which you then connect to all the others through interconnecting pipes (that bit is so simple even I can do it!) You can buy the necessary from a place like Wickes or B&Q but if you employ a tradesperson, they will buy it for you.
Ours is rigged up to collect water from the roof of one side of the house and we have two butts totalling just under 100 gallons under a carport.