Well, Airfix models are called:
Plastic Model Kits.
There are many websites on the subject, just google 'Airfix Forum', or something like that.
Here is a brief word how:
The model is made at the factory on a plastic sprue, sometimes called a tree. You cut the parts off the sprue with a scalpel, craft knife, or small pliers. Then you sand or file the rough edges off the part.
Following the instructions included in the kit glue the parts together with plastic glue. Some beginner kits come with glue and paints.
Then you paint the model in the appropriate colour denoted by the instructions, e.g. Airfix 78 is a grass green. There are two choices of paint:
Enamel - More choice of colour, more readily available, but as a solvent, poisonous. Acrylic - Cheaper, kid safe, but less available, and less professional.
Then if you want give the model a coat of varnish.
This is not a complete guide, just a starter tutorial.