When creating a weather forecast how do you determine the location of isotherms and isobars to draw the lines?

Question

creating the daily weather maps.

Answer 1

I guess you are just asking about how to draw isotherms or isobars, and not how to do something else after, like determine the wind direction or relative speed?

If you are drawing isotherms and isobars, you basically connect the dots.

You'll have a map of stations, and each station will show the temperature (to make isotherms) or pressure (to make isobars).  

For example, all the places that have a temp of 20C, you draw a line to connect them.  If there's a station with a temp of 18C, and a station with a temp of 22C, then the isotherm line gets drawn right in between.  You just connect the 'dots' the best you can.

Answer 2

based on temperature and wind readings of the weather stations. All the data from every station in Canada is fed to one source which constantly updates the location of fronts etc...