To measure angles:
1) Switch on the angle tool (click twice to make many measurements)
2) Click and drag up one arm of the angle from the vertex - as if drawing a line
3) Release the mouse
4) Click at the end of the other arm of the angle

To measure distance
1) Switch on the distance tool
2) First add a callibration measurement along an object of known length...
3) Click and drag along it - as if drawing a line
4) Right click to set the callibration distance
5) Now you can move it out of the way (but don't change it or delete it)
6) Now you can use the distance tool to make measurements

The timer tool is used like a stop watch: everything you would do with your thumb, you do with a right click: reset to zero, start and stop... all with the right click!

To help get accuracy of measurements:
1) Make the line thickness of the measurement tools zero pixels - to help accurate placement
2) Zoom right in on the obect being measured

Also remember that measurements on video are subject to lens distortion at the edges and this section of the video gives some basic advice on how to estimate error margins.