Cut a large rectangle of silver foil and a slightly smaller rectangle of parchment paper. Enough to go over the top of the pudding basin with plenty to spare. Then lay the foil flat on your surface and the parchment paper flat on top of the foil. Make a pleat in them both as shown in my video above.
If you need to see the technique more clearly have a look at this video
(don't worry about buttering the parchment paper like he says. though).
Cover the top of the pudding basin with the foil and paper making sure the pleat is in the middle, then press it down all around the basin as tightly as you can, then tie twice around the basin with string as tightly as you can. This is shown clearly in the video link too.
If you are using a plastic pudding bowl with a lid, I still recommend covering the pudding with a circle of parchment paper, putting on the lid, then wrapping the whole thing in foil as I have had situations in the past where the lid blows off and the pudding is ruined by the moisture.