Do not shrink from the trouble and expense of seeing the work in situ, and then, if necessary, removing it for correction and amendment.
If you have a large window, or a series of windows, to do, it is often not a very great matter to take a portion of one light at least down and try it in its place. I have done it very often, and I can assure you it is well worth while.
OF MAKING A SKETCH IN GLASS.
But there is another thing that may help you in this matter, and that is to sketch out the colour of your window in small pieces of glass—in fact, to make a scale-sketch of it in glass. A scale of one inch to a foot will do generally, but all difficult or doubtful combinations of colour should be sketched larger—full size even—before you venture to cut.
Work should be kept flat by leading.
One of the main artistic uses of the leadwork in a window is that, if properly used, it keeps the work flat and in one plane, and allows far more freedom in the conduct of your picture, permitting you to use a degree of realism and fulness of treatment greater than you could do without it. Work may be done, where this limitation is properly accepted and used, which would look vulgar without it; and on the other hand, the most Byzantine rigidity may be made to look vulgar if the lead line is misused. I have seen glass of this kind where the work was all on one plane, and where the artist had so far grasped proper principles as to use thick leads, but had curved these leads in and out across the folds of the drapery as if they followed its ridges and hollows—the thing becoming, with all its good-will to accept limitations, almost more vulgar than the discredited "Munich-glass" of a few years ago, which hated and disguised the lead lines.
You must step back to look at your work as often and as far as you can.
Respect your bars and lead lines, and let them be strong and many.
Every bit of glass in a window should look "cared for."
If there is a lot of blank space that you "don't know how to fill," be sure your design has been too narrowly and frugally conceived. I do not mean to say that there may not be spaces, and even large spaces, of plain quarry-glazing, upon which your subject with its surrounding ornament may be planted down, as a rich thing upon a plain thing. I am thinking rather of a case where you meet with some sudden lapse or gap in the subject itself or in its ornamental surroundings. This is apt specially to occur where it is one which leads rather to pictorial treatment, and where, unless you have "canopy" or "tabernacle" work, as it is called, surrounding and framing everything, you find yourself at a loss how to fill the space above or below.