First, which star, landmark and horizon field I will choose?
The star/landmark/horizon to choose is on the flight plan. I don't recall any landmark sightings used on P23, maybe on Apollo 7 but I can't remember. Apollo 8 onwards only uses horizon sightings during the translunar coast.
what is the difference in a landmark and horizonmark and where do I find horizonmarks
Landmarks are points on the surface of either the Earth or Moon used for sightings.
There are 2 types of horizon used in P23, near or far horizon.
If you imagine a straight line from the star passing through the center of the earth/moon, the near horizon is where the line intersects with the horizon, the far horizon is on the opposite side of the body.
White dot is the near horizon. Pink dot is the far horizon.
I am also expected to carry out a superimpose, star and landmark. How should I do this?
Ensure you have a proper IMU alignment first, otherwise use P51 and P52 to align the IMU.
When entering P23, enter the star, landmark, horizon that will be used for the sighting.
V05 N70
R1 000AB
R2 XXC00
R3 XXCE0
AB = Star code
C = Earth (1) or Moon (2)
E = Near horizon (1) or Far horizon (2)
If you are using landmarks, R3 must be set to 00000. If using a horizon, R2 must be set to 00000.*
Hit PRO on the DSKY.
If you're using a horizon sighting. (R3 not 00000) skip this:
V06 N89
R1 +XXXXX (Latitude of landmark)
R2 +XXXXX (Longitude of landmark divided by 2)
R3 +XXXXX (Altitude of landmark)
Enter all the info and hit PRO
V50 N25
R1 00202
R2 (blank)
R3 (blank)
Hit PRO
V50 N18 (I'm sure you know how to let the AGC perform an auto maneuver, so I'll skip this part)
Once the CSM is in the correct orientation, hit ENTR.
V51 N(blank)
R1 (blank)
R2 (blank)
R3 (blank)
Zero your optics and take a look through the sextant, make sure it is pointed at whatever landmark or horizon you're taking sightings with. Hit MARK.
V06 N87
R1 (blank)
R2 +XXXXX (shaft angle)
R3 (blank)
Hit MARK again to update R2 if required. Otherwise, hit PRO.
V51 N(blank)
R1 (blank)
R2 (blank)
R3 (blank)
Switch the optics mode to CMC and allow the optics to slew to the star used for sightings. Switch to the sextant view and hit 'v' on your keyboard. You should see the view rapidly switch between the earth/moon and the star used for sightings. Use the WASD keys to move the view such that the star and the landmark/horizon are on the same part of the scope. (They don't have to be in the center of the scope. They don't even have to be on the scope for that matter...)
Blue circle is the body used, white dot should be where both the star and the landmark/horizon is.
Hit MARK.
V50 N25
R1 00016
R2 (blank)
R3 (blank)
Hit REJECT to return to the previous page and take the mark again. Otherwise, hit PRO.
V05 N70
R1 000AB
R2 XXC00
R3 XXCE0
Ensure the info is correct. Hit PRO.
V06 N49
R1 +XXXXX (delta R)
R2 +XXXXX (delta V)
R3 (blank)
Same page as on P22. Hit PRO to update state vector.
V37 N(blank)
R1 (blank)
R2 (blank)
R3 (blank)
Thats the end of the program. Hit 00 to return to P00.
EDIT:
The last picture is not 100% accurate. In order to find the substellar point of the Earth/Moon to the selected star, you'll want to place the star on the top or bottom tangent point of the body. (Either the highest part or lowest part of the body when looking through the sextant depending on whether near or far horizon is used)