Meteorologists use observed pressure conditions to plot isobars. When isobars are plotted, the map starts out with lots and lots of pressure readings from lots and lots of weather stations across an area. When plotting isobars, a meteorologist will start at the lowest (or highest) pressure value and trace a track where that pressure exists on the map. The since isobars never just "end" in the middle of a map, the isobar will continue until either hitting the edge of the map or connecting with itself again (circular isobars that appear around pressure centers). Once one isobar is completed, they move on to the next value up (or down). The amount of isobars showing up on a given map depends on the pressure difference between the isobars being plotted. If an area is plotted using an isobar every 4 mb apart vs ever 8 mb apart, there will be twice as many isobars being drawn on the 4mb plotting vs the 8 mb plotting. Once the meteorologist goes through the entire range of reported pressure readings, the map is complete.
Obviously, nowadays meteorologist don't do this by hand very much anymore. Computers can calculate EXACTLY where each isobar needs to be located by differentiating the pressure readings across every single square inch of an area but, most meteorologists do have to learn how to plot by hand in their schooling.
As for forecast plots, they are generated by various computer models that use basic fluid movement (momentum) equations to predict where and how the weather will change over time. Once these models predict the pressure readings across an area... the whole process repeats itself.
Plotting isopleths (lines of equal quantity... pressure, temp, humidity, height, etc.) is not an exact science because no one can REALLY know what the EXACT pressure is over an entire area however, isopleths are used by meteorologists to see how a certain quantity is distributed over a given area. Ask 3 meteorologists to plot isobars over the same area and you will get 3 different renditions of the same map but overall, they will show the same major trends (a trough here, a high pressure there, etc.).
Here is a cool site that allows you to practice plotting your own isobars in an area. You can trace your own lines and then reveal what the actual conditions are.
https://www.e-education.psu.edu/file...tool_p0207.swf
Enjoy