By guess and by golly tutorial for dress or blouse:
Cut off the waistband. These skirts are often cut horribly off grain, so you lose a lot of fabric if you tear it. I choose to celebrate the wonkiness and maximize the fabric.
Hold the skirt up against your body and decide how long you want the blouse/dress/new garment.
Put a pin at the top of your shoulder.
Consider how full you want the new garment. Measure (i.e., wrap it around your body) and mark how big you want it at bust and waist. You may want to use the whole width.
Tiered skirts get wider as they go down; don’t cut straight down, match up top and bottom edges, folding the skirt along your cut line.
Cut off and save the extra fabric.
Use a pattern to cut the neckline and shoulders (placing the top of the shoulder seam where you placed the pin earlier) or to make a yoke, if you need more length.
I made a pattern from an existing blouse for this, but a commercial pattern is fine.
Stitch shoulder and side seams, finish neck and armhole.
If using a yoke, finish the neck and armholes before adding the body, so you don’t have to wrestle all that fabric. Alternative yoke: cut the bottom off a blouse.
If needed, do a gathering stitch around the top of the body.
Match up side seams and center front and back, pin and stitch. Check for mistakes and fix them.
Hem if needed.
For some blouses, I made a yoke pattern that’s long in back and short in front, to get a hi-low hem. The body piece is the same length all around, so I don’t lose any pretty hem details.