The simple Mitre joint is used to join components at an angle. It relies purely on the end grain glue joint, and is therefore not very strong. Often the resulting 'frame' is applied to another structure that gives it strength.
The two components are overlaid at the desired angle, and clamped.
Knife lines are made at both the intersections,
on both of the components.
Each component then has two knife lines that define the entire joint.
These knife lines are then joined across the joint diagonal with a third knife line.
This diagonal knife line defines the joint line, and the material towards the end will become the waste.
Create a knife wall by paring from the waste into the knife line.
Use a try square to guide knife lines down the edge of the joint.
Then saw away the waste.
The mitred joint should be at the desired angle.
To clean the joint prior to gluing (of fitting dry), use a shooting board with a fence clamped on at the appropriate angle.
The joint should now fit nice and tight.
If gluing, the joint surfaces should first be primed with a thin coat of glue, before gluing up and clamping.
CA (Super) glue can be used for small joints. In which case it should be assembled on a flat surface, with the joint just hanging off the edge, and held until cured.