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First off you MUST map your bombsite keys in your control setup. My suggestion is to use the numberpad keys located to the right side of most modern keyboards as they have no function (IF you are using a joystick with a hatswitch to "look around"). If not then any available keys will do, your choice.
Second, make sure you have the "auto level stabilizer" control mapped also (NOT "toggle auto-pilot" that's another thing altogether) this is critical in assisting with the bomb run using the bombsight.
Okay, you've mapped your keys, you're in your bomber, now what?
1. Set the bombsight to the altitude and speed you intend to bomb at. This can be done (try and get into the habit) prior to takeoff or well before reaching the target using your bombsight keys. For example you want to bomb your target from 10,000ft and intend to fly at 220mph. Using the Increase/Decrease Bombsight Altitude keys, set the alt to 10,000. Using the Bombsight Velocity keys set your approach speed (see NOTE ABOUT SPEED below). You will see the numbers indicated onscreen. You can change the viewing angle of the bombsight by using the Increase/Decrease Bombsight Distance keys (I always set this to 80 degrees as it will allow you to see the target in the distance vice looking straight down, you can decrease the angle as you approach the target so don't worry about it).
OK, you're in the air and on approach to your target, now what?
2. As pilot, line up the plane with your target. Engage the "auto level". Press the "C" key to go to the bombardier's position then "Shift-F1" to put your "eye" to the bombsight. You will now see through the bombsight to the target area. You should see illuminated crosshairs also, unless the plane is still settling down from a turn in which case it will take some time to appear (the crosshairs will disappear/unlock if the plane is moved more than 4.5 degrees during a bombrun therefore SUBTLE movements only during the bombrun). If your target is still in the distance and approaching you can move the crosshairs up or down onto the target using the Increase/Decrease Bombsight Distance keys. If the bombsight is lined up with the target and centered then hit the Toggle Bombsight Automation key to "lock" and you will see that the crosshairs now stay with the target. If the crosshairs are drifting vertically off of the target then use your Bombsight VELOCITY keys to adjust it.
If the crosshairs are NOT lined up (i.e. you must move them horizontally) then you have a couple of options (in my opinion);
a. If close to the target, at the bombardiers station you can unlock autolevel and using subtle movements of the joystick to get the plane lined up and keep the crosshairs visible. Once happy then relock autolevel and resume your bombrun. NOTE: you can use rudder trim also to make fine adjustments instead of the joystick. This takes time, practice and patience. OR
b. If you are far enough away and have the time go back to the pilot position unlock autolevel and make corrections from there (use your PDI and radio compass to assist you in getting right on course). Reset autolevel and go back to the bombardier's position and resume your bombrun.
If you've managed to set the crosshairs on the target and "locked" the bombsight on then continue to watch the crosshairs to make sure they stay on the target and the bombs will automatically release at the appropriate point and you will see a "bombs away" display and the plane will "lurch" as the extra weight is suddenly released. Unlike B17II, there is no "indicator" to tell you how close you are getting to "bombs gone", the "bombs away" message is it!
3. If everything is done correctly you should have the satisfaction of seeing your bombs hit the target (F7 should give you the target view).
4. The left/right sight control is to adjust for wind 'deflection'. It takes practice and a long bomb-run to be able to adjust left or right to compensate for 'drift'. If you have wind speed set to zero then you don't have to worry about this. If you have wind speed set to something other than zero, the previous steps still apply but you use the right/left adjustment to get the sight to run 'straight'.
NOTE ABOUT SPEED: The speed you are setting into the bombsight is NOT the speed you are seeing in the bottom left of your screen which is the INDICATED. You are setting the TRUE airspeed. Here we have a slight dilemma in that you can check TRUE airspeed using the Ctrl-F1 to cycle through the various No-Cockpit views when offline but many times online this can't be done as it's been disabled by the host so you are left to guess. A very GENERAL rule is to use the KIAS airspeed as that is close to TRUE airspeed or is close enough that only slight adjustments on the bombrun are necessary.
Now, you're done, right? Nope, not if you want to go further. If you are nice and comfortable with bombing as suggested above you will have noticed that there is no way to adjust the bomb interval if you use the auto-lock (i.e. hit the bomb switch and have just the first two bombs go, wait a couple of seconds then hit the bomb switch for the next pair and so on). There are "bombing tables" (i.e standalone mini-mods which will calculate intervals for you at given heights and speeds) for manual bombing. This is essentially repeating the procedure above without using the Bombsight Automation Key but releasing the bombs manually yourself. If you search M4T or SAS I believe they still have bomb tables program (or at least charts).
This link has more detail than you probably need at the moment and includes my instructions above.
https://www.sas1946.com/main/index.php/topic,52863.msg575185.html#msg575185