I put the four squares together like they would be sewn on the cutting table and measured. I then cut out a square and inset it. It just looked wrong. The points of the star didn't match the points of the neighboring stars. Why....oh, the square is too small. But what do I do? The measurement was right, I thought. The seam allowances were messing me up, so I finally ended up constructing paper octagons of the correct size, taping them together and measuring the square hole left. I had made the square 1/2 inch too small. I finally got the right measurement and I'm on my way.
Here is the back side of what I've completed so far. I've outlined the squares that have to be inset in blue so they are more clear. Once I get them together it grows pretty fast, but it took sewing and un-sewing a lot before I got it right.
Oh, you might also be wondering (if you noticed) about my clever use of the plain octagons to set off the pieced ones. Why, you ask did I make that design decision. Well...if you stack fabric to cut these octagons, you need 8 layers. The fabric repeat was about 12 1/2" and the strips for the kaleidoscopes was 6 inches I got 12 sets of triangles to sew into octagons from each stacked set. That made 24 blocks. No problem, that's plenty.
Unfortunately I don't seem to be able to count well in the twenties, I needed 26. So, use another 3 yards of fabric for two blocks or...get creative? You can see I decided that it wasn't worth the extra 3 yards. I think I might move the lower plain block to the right one space, but I'm not sure yet.
In case you're interested, here is how I constructed the octagons. I knew I wanted them to be 11 inches finished, so I drew a circle of 11 inches. Then I folded it in half (just folding across the pencil line at the edge of the circle, I then drew a line between those two marks and did that three more times. I used one of my acrylic rulers and put the edge right at the edge of the circle with one of the perpendicular lines on the drawn line. I drew a line and around you go. It worked really well to get the right size octagon.
Happy quilting and may all your seams match.