This is a general technique used when sewing fabric edges together which have two different curves, or when sewing a curved and a straight edge together.

The problem is that we usually sew seams by matching up the edges of the fabric. But when the two edges have different shape curves, then the cutting lines are different lengths.
The stitching lines are the same length, so it is those that have to be matched.

Some people manage to deal with this simply by ‘walking’ the fabric pieces together along the stitching line, by feeling along the stitching line with their fingers.

If you’d prefer to use the ‘proper’ method :
This link, princess seams, goes to a photo tutorial from Megan Nielsen patterns.
In summary :
staystitch along the shorter edge,
– clip through the shorter seam allowance, so it can stretch out to match the length of the longer edge.

Similar methods for stitching straight edge and curve :
Straight edge and inward curve, such as joining collar to neckline :
staystitch and clip inner curve of the neckline, so it can fan out to meet the longer straighter edge of the collar or neckband.
Straight edge and outward curve, such as sewing sides of bucket bag to bottom of bag :
staystitch and clip straight edge at bottom of sides of bag, so it can fan out to match the longer curved edge of the circular/ oval bag bottom.

The general principle is to staystitch and clip the shorter edge.