The tips found here are all based on my reading and use of the excellent how-to book: "Mosaic Picture Quilts", by Pat Durbin and what I learned as I made my own mosaic quilt. Before doing yours, you should probably refer to the book and my post Sunflower Mosaic.
1. You can get 3 different sizes (scales) of transparent grids from Pat's web site. Use the appropriate template for the piece you are doing. The original photo or print of art work does not need to be the same (full) size as the template. Tape the transparent grid in place over your original. I also made a small "porthole" by cutting a square (the same size as the squares in the transparent grid) in a piece of paper which was about 3" square. This allowed me to isolate a single square on the original so that I could truly see the colors and shapes contained therein.
2. Draw a grid of 1" squares (or the appropriate scalable size) on a large sheet of paper to the size of the finished quilt top. Make sure that you leave enough extra room all around so that the binding will not cover any important part of the pattern. Then, on each side and top and bottom, mark off all of the rows and columns the same way they are marked on the transparency. This will help you later when you are trying to find what square is "R12C11" on the enlarged grid.
3. Cut light-weight non-fusible interfacing a little bigger than the gridded paper. Tape it flat around the edges.
4. Cut strips of light-weight (e.g., Steam-a-Seam II Lite) fusible webbing 1/8" bigger than the drawn grid squares. So, for example, if your grid had 1" squares, your strips will be 1 1/8".
5. Cut strips of appropriate fabrics the same size as the fusible strips x width of fusible webbing. Cutting on the bias will minimize raveling and stray threads.
6. Fuse webbing to the backs of the fabric strips. Note: You may prefer to fuse a sheet of webbing to a section of fabric before you cut it into strips. Either way works well.
7. Cut squares of the fabric.
8. Starting from any corner, work in a diagonal to fill the gridded space.
9. Peel backing from the squares (a tweezers really helps) and press into place. Each square fits in a grid corner and slightly overlaps the 2 adjoining squares.
10. After placing a few pieces in place (or even just one) and ensuring that they look ok, fuse it in place per the instructions. I use a piece of muslin between the iron and the square to avoid getting any melted webbing on the iron. This also helps keep "heat shine" from happening to the fabric. Be careful as you fuse near the edges not to melt the scotch tape that is holding the interfacing down. As you go along, you may also find that some of the fusible web has actually gone through the interfacing and fused to the gridded paper. Not to worry, this is a good thing! It helps keep the interface in the correct place as you build the top, and is easy to peel away once the top is complete.
11. For grid squares that require several pieces of fabric, I recommend this:
a) Place a piece of the Steam-a-Seam paper from a previously used square in the appropriate spot on the grid.
b) With a pencil, lightly sketch the lines that make up that piece. For example, the top section might need a green fabric and the bottom a blue fabric (there can be more than 2 in a small square!).
c) Using that small square as a template, cut the pieces from the indicated fabrics. You can either cut each individual piece (to then fit together to make a square) , or, use one fab as the full square base and just cut the balance from the other fab. Doing this means that there will be no gaps of interfacing showing through if your cutting in not exactly perfect. But, if you do this, BEWARE: The lighter fabric should always be used as the "base" square, even if it is the smaller of the sections in that square. Why? If the darker fab is the base, it will tend to darken the color of the lighter fabric placed on top of it. So then that fabric's color (value, intensity) may not match those around it; it might stick out inappropriately.
12. When the whole top has been created / fused:
a) Look for any areas where there is an abrupt (and inappropriate) break in the pattern. This usually happens at the edge of a square. Find a piece of the matching fabric and free-hand cut a small "rounded" piece to soften the transition.
b) Go back over it lightly with the iron once more to ensure that there are no unfused edges.
13. Make the quilt sandwich with batting and backing as usual, but do not pin as you normally would. Once there is a pin hole in a fused square it will not "heal" (like a plain piece of fabric would) when the pin is removed. I basted across the bottom, quite close to the edge, and then a few inches up each side. This basting line was inside the area where the binding would go. I also put a few straight pins along the top edge (and parallel to it).
14. Quilt as you normally would, but use a thin (70/10) needle.
15. Pat uses a tulle overlay on all of her quilts as the fourth part of the sandwich (and quilts it into place). I bought a piece, but did not use it in mine for these reasons:
a) It was very slippery (as Pat notes in her book) and I did not want to put pins in my top to hold it in place.
b) Even though I bought a white piece, it covered and dulled the colors in my top.
c) Though it seemed sturdy, I was very fearful of somehow tearing it while quilting. Worse yet, was what would happen if, for some reason, I had to rip out a section of quilting (as indeed I did due to a tension problem!). Could I actually rip the threads without tearing or making a mess of the tulle?
It seemed like the risk / reward was not in my favor.
16. Bind as you normally would.