You can use it in one of several ways...

I have a HW100 so I use the U shaped wire clamp to attach it to the silencer.

I could lay the rifle in the V shaped trough with the chronograph on a table and shoot through it that way.

If your rifle has just the barrel then the chronograph will probably attach using the V shaped clamp top and bottom.

If after switching on the chronograph it displays FPS it is basically asking the question do you want to work in FPS. If yes immediately press the function button else it will work in MPS, metres per second.