The professional way is to tie off the first balloon, inflate the second and knot it directly to the neck of the first balloon. However, you may find that you lose air as you do this. Alternatively you could knot both balloons and then tie them together. There really is no easy way to do this, but with practice it will become easier.
Now cross the two pairs of balloons (fig. 4), and then twist them around once, so that you have both colors directly opposing each other (fig. 5). This first cluster can now be put in position at the base of your stand (fig. 6).
Position the balloons so that the conduit is between two of them and simply push the cluster onto it, until it's in contact with the knots in the middle. You'll find that it's such a snug fit that the balloons are self-securing. This technique will be used for all the clusters of the column.