Then they found all the pieces that had a straight edge on them and fitted them together - these eventually made up the outside frame of the picture. Then they tried to fit the other pieces within that frame. Sometimes it took ages to find just the right piece, because all of the sides where they meet had to fit together.
Once they had spotted a likely piece, and managed to fit it in, then they moved on to the next piece. They each tried to do a particular area of the jigsaw because otherwise it was very difficult to see what they were doing - arms and legs and paws and heads everywhere! This didn't always work though!!