When do I inc 1 st?

I’ve started the “Deramores Waistcoat” pattern.

**Using 6mm needles, cast on 23sts and P1 row.

[B]Shape front edge[/B]
Starting with RS knit row, work in St st [B]at the same time[/B] inc 1 st at the front edge on the next 5 rows. [I]28 sts[/I]

Basically, when do I inc 1 sts at the start of the row or the end?

Since this is the right front part of the waistcoat, that’s right side when you are wearing it, start the increases at the beginning of a RS row. That’s the front edge. The other edge is the side seam edge.
The directions say to increase on the “next 5 rows” so that means increase at the beginning of the RS row and then the end of the WS row. Continue like this until you’ve increased 5sts.