The picture looks like simple miter but how can I make this a strong miter myself?
From my view, the easiest way to make this angle miter stronger is with pocket hole screws and glue. The pocket hole setup only requires the drilling to be done on one piece. After the holes are set, clamp the two pieces together and insert the screws.
All the normal low price jigs for the other joints are designed for 90^ or straight on joints.
You could also use the pocket hole jig and create a dowel joint by drill through both pieces from the bottom. This would require some setup time to determine and shim the guide to obtain the correct angle. After the hole is cut, insert dowels the normal way.
Added illustrations of doing either the pockethole or dowel to strengthen the miter joint.
The indicated joint could be created using mortise and tenon, dowel pins, or biscuits. I suspect the level of strength is in that order as well.
It's less likely that this joint is connected using pocket screws, but you could also accomplish a strong connection in that manner, and fill the screw holes.
Gluing is implied in all cases.