If you want to fill out your sleeves you will do the close grip bench. Your triceps make up 2/3 of your arm. Sure your biceps get noticed first, but the true “damn dude!” comes from a cascade of flesh pouring out of your sleeve. And the close grip bench press is the best way to overload your triceps.
You will probably be able to work with 70-80% of your flat bench work load, because this is also a compound movement. Start off with the same body position as the flat bench: back arched, shoulders pinched back. Grab the bar at your shoulder sockets. Your goal is to lower the bar in such a way that your elbows stay parallel to your body, straight up and down. Where the flat bench goes down to your nipples, the close bench goes to your sternum. As you press up, really focus on your triceps extending.
When you’re finished, get up and beat your chest like a gorilla.