Guide
Pipe puzzles: start from the edges
Corners and dead ends have the fewest options.
A pipe on the border can never point outside the board, and a corner tile can only face inward. Fix those first.
Dead ends (tiles with one opening) next to each other can never face each other, or they would close a tiny network. Straight pieces on the border must run along it.
Every tile you settle removes options from its neighbours. Work inward from the edges and the middle solves itself. The network may never form a closed loop.