You have the power switch at the bottom right. Bottom middle is A "auto" and M "manual"
"Ready" (red) shows when fully charged (the LED in the viewfinder should also light when ready).
"Load" (is charging, batteries strong enough to light LED but recharge time of 30 to 40 seconds means batteries are going)
"Reset" (when flash goes into power saving mode you need to only press this to wake it up)
You have a Low and High switch, this will allow a bigger charge to be released and can use more battery power. As you move the switch to Lo/High you will see the distance scale increase.
On the left you have a slide switch. That is for
ASA of the film in the camera from 25 to 1000.
So if you set that scale to ASA 400, the blue line with the green LED shows F-11. So you set your lens to F-11 and fire away from 1.5 feet to 70 feet ( as shown on the bottom of the scale) on "High", only to 17 feet on "Low".
The flash also has a diffuser, top distance will go down to lets say 40-50 feet for the same film speed but allows a 20mm lens.
At the top is Audio check, if it is turned "ON" and you fire the flash (top right is a TEST button) and there is enough light for a correct exposure, you will here a slight musical sound.
TTL mode: Set the camera XR-P or XR-3pf to A or Program
and the lens to A. Start flash and fire away. If you choose
a different f-stop the flash will compensate up to a point. Check
the XR-P manual.
In TTL mode the light from the flash is determined by the meter IN THE CAMERA. Hence a telephoto lens will not be fooled by reflective light of close objects, only the light coming from the image in the viewfinder.
Manual mode: choose the ASA of the film on the slide dial,
look at your foot distance and follow that line up to the diagonal yellow
line then left to the F-stop on the right. You can choose High or
Low again. At ASA 400 on "high" at a subject 17 feet away you would
choose F-16. At 8.5 feet you would choose F-22. Hence the Auto
where you choose F-11 and let the electronics take care of everything.
260P: Same idea except you have a TTL switch in which you set the camera to a program mode/Auto and the lens to A. In Auto, follow the switch's green line to the ASA setting that must match the ASA of the film in the camera.
Note the "A" , TTL and M settings. This has one F-stop ability
as the 300P and a Test light.