Many woodworking books and YouTube videos have been made about jigs and fixtures.
The job is not difficult for lack of a planar surface but made difficult by lack of clarity of thinking.
Put the drill point roughly in the centre of the break (draw two diagonal pencil lines if necessary) and as square in two directions as you can judge and have at it. Pick the larger and flatter of the two pieces. Once the hole is drilled use a dowel point to mark the second hole. No dowel point? Drill the first hole real small (1/8"), push in a nail and clip off the head.
As above, an oversize hole and epoxy filler will allow the dowel to self-align to overcome any errors. Do not overstress about lack of precision to the point of inaction but use methods that can accommodate lack of precision and still achieve a good result.
If you are not confident you can drill straight, temporarily clamp the two parts together. Stick masking tape over the joint. Draw a line on the tape extending onto both pieces, Slice the tape roughly on the break. Use the tape as a reference plane and the line as a reference line to locate a standard dowelling jig (https://www.diy.com/departments/silverline-dowelling-jig-30mm/5055058124845_BQ.prd). Masking tape not big enough? Use a cereal packet as a larger canvas for your lines. Cardboard not stiff enough? Use a piece of 3mm plywood.
A modern (though pointless) solution would be to 3D model each half of the break and then model a two part alignment guide that will slip over each part, through which the drill bit will pass, guaranteeing a perfectly aligned hole.
The metal dowel is a good safety measure as any future failure of the joint would visual and gradual, not catastrophic, thus preventing your Great Aunt Maude being dumped on the floor.
The boy stood on the burning deck; his face was all a quiver. He gave a cough, his leg fell off and floated down the river.
