Sewing the curve
Fold the pie and the background in half on the curve, or use the center notches, and pin them right sides together at the middle and both ends. Sew with the background on top, easing it round the curve a little at a time and keeping the needle down when you stop to adjust.
How the template is drawn
A seam allowance on a curve is a curve with the same center: the pie is cut at the radius plus the allowance and the background at the radius minus it, so both sewing lines are the same curve. Freshly Pieced's 3 in drunkard's path template is drawn the same way.
Example: a 6 in unit with a 4 in curve and 1/4 in seams. The pie is a 4 1/4 in quarter circle cut from a 4 1/2 in square, and the background a 6 1/2 in square with a 3 3/4 in quarter circle cut away. Sixteen units take two 4 1/2 in strips for the pies and three 6 1/2 in strips for the backgrounds.
Questions
What size is a drunkard's path block?
Any size you like. Four units make a block twice the size of one unit. The curve radius sets how big the circles look: the closer it is to the unit size, the thinner the background.
Why is the pie cut bigger and the background curve smaller?
Each piece needs its seam allowance outside the sewing line. On the pie that is outward, so its curve grows by the allowance; on the background it is toward the corner, so its curve shrinks by it.
How do I sew a curved seam without puckers?
Put the background on top, pin the middle and both ends, and sew slowly, easing as you go. A shorter stitch helps on small curves. Clip the background allowance only if the seam will not lie flat.
What are the notches for?
The long notch marks the middle of each curve, so the two pieces line up before you pin. The short ones mark the quarters, which helps on curves over about 4 in. Snip them no deeper than half the allowance.
Can the curve go all the way to the corners?
Yes. Set the curve to the unit size and the background narrows to a point at each end. That background is also the one used in a pieced orange peel block.
How do I know the printout is the right size?
Measure the square at the top right of each page: it must be exactly 1 in (or 25 mm). If not, print again at 100% or Actual size, not Fit to page. From a phone, save a PDF and print that at 100%.
Sources: Freshly Pieced, Drunkard's Path Block Basics (WeAllSew).