There are several places one can look online to find the formulas required to manually determine the best-fit line for a set of data points in a plane. If you are having trouble understanding how to use the formulas, it would be easiest to set up a session and go through the process together, rather than for me to post a bunch of calculations, which wouldn't likely not make a lot of sense if you don't understand the process anyway.
In practice, since the process is formulaic, it can be automated, and most modern scientific calculators have regression features that can be used to generate the equation of the best-fit line.
