When should I overlock — before or after?

The order depends on the fabric, and getting it wrong costs you the ability to adjust the fit.

On woven garments: sew first, overlock after. Sew the seam on your sewing machine, check the fit, press it, then overlock the raw edges. This keeps the seam accurate, keeps the allowance available if you need to let it out, and lets you try the garment on before anything is trimmed away.

The exception on wovens: overlock first when the seam will be pressed open and you want each allowance finished separately — trousers, lined jackets, anything where the inside is visible. Finish each cut piece before construction.

On knit garments: the overlocker usually does both at once. A four-thread overlock sews the seam, trims the allowance and finishes the edge in one pass. That is the whole point of the machine on jersey, and there is no separate finishing step.

But baste or sew first if fit is uncertain. Once the overlocker has trimmed, the allowance is gone. On a first version of a pattern, sew the seams on your sewing machine with a narrow zigzag, fit the garment, then overlock the seams once you are satisfied.

Some seams are always sewn first. Anything needing pressing open, anything with a zip, anything requiring precise matching at a point, and anything you may need to adjust.

And finish before you attach. Hems, facings and pocket edges are easier to overlock while the piece is flat than after assembly.