The most basic rules are these:
If it’s too long, cut it off
If it’s too short, add to it
Scissors work great, but I prefer the rotary cutter when slicing angles or large shapes (for instance, when I’m working with roof and sky pieces).
During the early phases of construction, there is no proper size. Nothing has to be just right, therefore, choose how you want it to look. Add or subtract, it doesn’t matter, but keep in mind that you will be losing ¼” in the seam allowance all the way around.
A consistent ¼” seam allowance isn’t necessary. Sew in a straight line and have enough fabric in the allowance that your seam isn’t going to shred apart.
Inserting a strip means slicing a cut through a piece of fabric and sewing a strip of fabric into that gap. For example, in this block, a strip of dark blue fabric has been inserted into a turquoise square:
I began by slicing the square down the middle:
Then I flopped one of the newly-made rectangles over on top of the strip and sewed it lengthwise:
I then sewed the other turquoise rectangle on and had the block I showed you up above.
Inserting at an angle is slightly trickier. This is the final block with an angled insertion:
I sliced across the original turquoise square --not going into either corner of the square--and then placed the strip in the gap. You want to sew to the square part that the top of the strip dips down to meet (in this picture, the one on the left).
Press the unit flat and THEN trim the strip even, following the lines of the original piece:Now add the other side of the square, again getting that little bump up at the quarter inch mark:Now when you insert a strip into a corner, you'll probably end up with extra bits of strip to slice off. Always follow the line of the base fabric and you'll be fine.
And you don't have to insert a single strip of fabric. You can make a strip by joining two different strips together and then insert them in the usual way:
Now don't ask me why, but when I did the next graphic, I changed the angle of the slice. With these next illustrations, I wanted to show that when you slice from corner to corner, you will get a different shape depending on where you sew the second triangle back on. You'll either get a square:
Or a rectangle:
Sometimes you want the strip insertion to be a little more complicated. For instance, this is how I used to use for make the letter y. First I visualized it (sometimes it helps to draw sketches or mark the fabric), and then I sliced, starting with the first long cut. I set that side apart and then made the second slice. I inserted a strip into the shorter gap first, making sure to trim it up even, and then I inserted a strip of letter fabric into that long gap. Trimmed it up and added that other triangle back on:
I used too big of triangles (and ended up with more of a square in a square) so I cut my block down smaller to make it work. Remember, if it's too big, just cut it down...
Here's how you would angle the longer triangle on and sew.
Say you want to match an angled fabric up so that you get this result:
Join them together, offset as always.
This is probably familiar to anyone who's used angled joins for their binding. (I did that once and then never again. Too much work - which is kind of a hoot when I think back on it now.)
Well this is the same process you use to make the angles on flat roofs for example. To get this:
And yes, you can start with a triangle to add a triangle. I often call that inserting a triangle, for obvious reasons.
To sum up: if it's too long, cut it off and if it's too short, add to it. Have fun.