To knit a waistcoat, ideally you need a pattern. This is because there are hundreds of different knitted waistcoat styles and each has very precise instructions. If you wish to knit a waistcoat without a pattern, you need previous knitting experience. This should include knowledge of basic techniques like casting on, how to knit a stitch and how to purl a stitch. See links in Resources for information on these. Get up to speed with these essential techniques first before starting to knit a waistcoat.
Knit the back of the waistcoat first, suggests Coats Crafts. Cast on 109 stitches for a small waistcoat. For a medium-sized waistcoat, cast on 123 stitches. Cast on 139 stitches for a large waistcoat. Knit the back as if knitting a cardigan.
Knit the left and right front sections next. Cast on 54, 62 or 70 stitches in each case. Knit each as if knitting half of the front section of a cardigan, including the neck edging.
Create the buttonhole band along the right front edge. Make this slightly wider than your chosen buttons. Use a ribbed stitch to make the band stretchy. Use a contrasting wool colour to make a feature of the buttonhole band.
Add the button band along the left front edge. Work it so it matches the buttonhole band, but without the buttonholes. Use the same stitch style and wool colour as the buttonhole band for a matching look.
Knit the armhole edgings. These will prevent the armholes from fraying and add style. You can use the same stitch style and wool colour as on the buttonhole and button bands if you wish.
Join the back and front pieces with joining stitches. According to Learn 2 Knit, you can use grafting, mattress stitch and other stitches for joining purposes. Aim to make the stitches invisible unless visible stitches are part of the design.
Sew on the buttons. These need to be on the button band and in line with the buttonholes. Check the buttons open and close properly before deciding the waistcoat is complete.