Okay, so I really got started good on the window for the bath. I'm going to try to explain this but I don't know how good I'll be at it. If I say something that doesn't make any sense, which wouldn't be unusual, just ask me to clarify.
You have to have a good, sturdy table that is at a good working height so you don't have to bend over. Old, solid core doors make wonderful work tables. Anyway, screw two boards to one corner, making sure with a framing square, that the inside corner is square. You will start the window in this corner and work outward. The boards should also project about an inch or more above the top of the table surface in order to accommodate the layer of ceiling tile and glass. Leave the other two sides open. Then you take your full size cartoon of your window and tape it down, making sure it is up against the boards and square.
The zinc U-channel is cut for the two sides first and put in place and then you start cutting the glass in that corner, working out. Most people make two copies of the pattern in order to cut one up for tracing on opaque glass. I just lay the glass in position, making sure it is fully in the appropriate channel, and trace the shape on the glass. If I use some opaque, I just use some tracing paper to get that one shape. You also have to decide if you are building the window face up or down, as the glass is heavily patterned on one side. Keep this in mind as you build so you don't cut a piece wrong side up! Yes, I've done that. I use a grinder to smooth up the edges and perfect the fit, then slide the glass in place and secure it with horseshoe nails, keeping it tight in that corner. These nails have a broad head and are easy to push into the ceiling tile. Also, they are flat on one side and will not roll around against the channel. You can use small pieces of channel (came) as this is temporary, like in the photo below.
So, you just keep cutting the glass and lead came a piece at a time, working your way diagonally across the pattern. I am using a few pieces of zinc came on the long, straight runs to help keep the window from eventually sagging. I am also using copper foil on some of the really wiggly edges because it would be about impossible to get lead came to bend like that. This will add a real organic feel to it. I also write the type of glass I'm going to use in each space to help me get it laid out and to plan ahead.
Hope this makes sense. If not feel free to ask more. I am going to try to get a lot done on it tomorrow; didn't get much done this weekend.