I would like to know how to record a demo on a specific lap, or laps. I want to record some demos so i can i watch them later.

press "2" after your lap, then name it and it will be saved in single player replays

Quote from diogosousa :

I would like to know how to record a demo on a specific lap, or laps. I want to record some demos so i can i watch them later.

Diogo não podes gravar uma volta específica, podes sim, gravar o replay e depois correr o mesmo até ao momento que desejas visualizar.
Para gravar basta carregares no 2, e "dares" o nome ao replay.

Now in english

You can't record a specific lap, but you can record a replay by pressing 2, and then play it to the specific moment you want to see.