Well done, thanks!

How do i start with a puzzle like this? all i can colour out for sure is 15/15 (from top left), how do i go on?

Row 1 col 12,Then diagonals to both sides to start.

Multiple possible solutions. Thank you for the hint Margpheb.

Lucky finding the right solution at 2nd try.

Continue marhpheb's diagonals until you hit each side, and join up with the 2 in the bottom row.