How did Harris and George get trapped in the canvas?

The friends intended to spend their first night on Magna Charta Island, but are too tired to travel all the way there, and decide to stop earlier. Because they did not bring a tent, they have to pitch the canvas cover over the boat before they can sleep. This task proves more difficult than it seems, and it takes them several attempts to successfully set it up.

In hanging the canvas, Harris gets all rolled up while trying to pass it onto J. on the other side. While struggling to free himself, Harris knocks George over, and they both inexpicably tangle themselves up and cannot move. 

George unrolled it, and fastened one end over the nose of the boat.  Harris stood in the middle to take it from George and roll it on to me, and I kept by the stern to receive it.  It was a long time coming down to me.  George did his part all right, but it was new work to Harris, and he bungled it.

How he managed it I do not know, he could not explain himself; but by some mysterious process or other he succeeded, after ten minutes of superhuman effort, in getting himself completely rolled up in it.  He was so firmly wrapped round and tucked in and folded over, that he could not get out.  He, of course, made frantic struggles for freedom—the birthright of every Englishman,—and, in doing so (I learned this afterwards), knocked over George; and then George, swearing at Harris, began to struggle too, and got himself entangled and rolled up.

I knew nothing about all this at the time.  I did not understand the business at all myself.  I had been told to stand where I was, and wait till the canvas came to me, and Montmorency and I stood there and waited, both as good as gold.  We could see the canvas being violently jerked and tossed about, pretty considerably; but we supposed this was part of the method, and did not interfere.

We also heard much smothered language coming from underneath it, and we guessed that they were finding the job rather troublesome, and concluded that we would wait until things had got a little simpler before we joined in.

We waited some time, but matters seemed to get only more and more involved, until, at last, George’s head came wriggling out over the side of the boat, and spoke up.

It said:

“Give us a hand here, can’t you, you cuckoo; standing there like a stuffed mummy, when you see we are both being suffocated, you dummy!”