There are several ways to make a weather forecast. Here's how we usually do it.
First, we take a "surface observation" (also known as taking a look outside). It's always nice to get the current conditions first-hand! We can tell a lot by the shape, color, and altitude of the clouds, the feel of the wind, etc. Next, we look at the satellite and radar to see where the clouds and rain are.
Next, we look at the computer models which show a specific forecast scenario. We compare all the various models, and then compare them to the current weather.
Finally, we take all those computer models, and our knowledge, and make a forecast. First we make a forecast for temperatures, then cloud cover, wind, and rainfall chances.