Lilli, I have just converted 4 blue plastic barrels (the type with only 2 inlets on the top,(screw-on/in used for concentrated food/orange juice etc.) at the top) into water butts, I obtained a standard water butt tap off of ebay, (type in water butt tap), they have a 26mm (approx) thread on them, I drilled a 25mm hole where I wanted the tap to be, filed a very slight chamfer on the tap thread, and a slight filing of the hole in the barrel, and gently screwed in (by hand), the tap, when it was half in, I smeared the remaining thread with Araldite, then screwed it in the rest of the way, making sure I stopped when the tap was vertical and the collar was surrounded by Araldite,(which I smoothed to a nice water-tight finish and left it alone for 24 hours),and did exactly the same for the barrel connecting pipe, (to connect 2 or more barrels together). Now I have 4 water butts without the large lid which can blow off,or allow debris in (+ 3 genuine butts). Either leave the screw on cap loose or drill a small hole in it, (5mm is plenty) to allow the air to escape when water goes in, if it is the second barrel. If it is a stand alone barrel, the inlet hole, (for the down-pipe connection was done the same way, this connector had the thread on the exterior, so I fitted it through the filler hole, (having made the hole slightly bigger than the thread of the connector), using a "mouse" to pull it through the hole, then put the nut on, not forgetting the rubber washer, but not using Araldite. If this makes sense.
. arh.
p.s. if it's a wooden barrel, google "knock in barrel tap", these come with the instructions, (usually), and if it's a metal drum, (45 gall oil drum for example), do the same as for plastic, but be careful. the hole edge is sharp. And a "mouse" is a length of springy wire with a loop on the end, which electricians use to pull wires through conduit. But any springy wire, (fence wire?) will do.