You need three pieces of information to fix the position of the aircraft you are seeing at a 3D-point for a given moment in time.
The following is not meant as practical exercise :-).
1) Azimuth.
Use a compass to get a bearing on your target.
You could draw a line on a map and know your target is somewhere above that line (or on it or even below it in the general case). So you have constrained position from anywhere in 3D-Space to one half of a 2D-plane.
2) Elevation.
Use a sextant to measure the angle from your eye between the horizon and your target.
Now you have removed another degree of freedom. You know your target is on a one-dimensional line. Extending from your eye, in the direction of the Azimuth and angled upwards at the elevation-angle.
For the third bit that finally constrains you to a zero-dimensional point in space you have a choice.
3a) Distance or "slant range".
Measure the apparent size of your airplane at unknown distance and the apparent size of a known object at a known distance. You could take a picture of the plane and put a ruler in it, or use a house that happens to be in it anyway. As you can also look up the actual size of the airplane you can calculate how far away it is along that slanted line.
You can convert elevation and slant range to find the distance on the ground and altitude. This is a bit of trigonometry. The slant range is the length of the hypothenuse in a right triangle where ground distance and altitude are the other two sides.
3b) Altitude.
Ask them for their altitude. I'd say you could use the data from the tracker, but as you are not trusting that it is not a real option for you.
If you do get an altitude (above or below your position), you know it will be along the slanted line at the point that happens to be at that altitude.
The math is about the same, only this time you know angle and length of the far leg of the triangle.
On a map you can now draw a point from you, in the direction of Azimuth, ground distance away, and label it with the altitude.
Do it quickly to sample several points, fit a nice path through them and you will find at that phase of flight it will be very close to a straight line for every successful landing. And also very close to the SAME straight line for different aircraft.