It just takes practice myroush. When you start off, just lightly feather the gas as soon as the clutch starts engaging. For normal driving, shift around 1500 rpm. The faster you want the car to go, the higher you take it up in the rpm band. You will probably wind up like most of us, which means driving around at higher rpm's listening to your car purr. Don't feel bad about this, it's not showing off, it's more like an addiction. lol
I find the key to smooth shfting with the Mustang, being that it seems to rev much more freely than the older muscle cars with manual trannies that I was used to, is to make sure you completely have your right foot off the throttle before starting the shift. Other than that, it's just timing, smoothness and practice. Once you feel confident driving her normally, you can gradually start being more aggressive with it.
Don't feel bad about making mistakes either, I drive tractor trailers for a living, for the last 20 years, and I still make the odd mistake. What's really funny is that in the big truck I have 15 gears, and reverse is in the truck is in the polar opposite position than it is in the Mustang, so I really have to watch that after a 3000 mile week in the truck. lol
Also in the truck, the big diesel automatically applies a bit of power when you engage the clutch from a standstill, meaning you don't have to use any throttle at all to get rolling, even on an uphill grade start. So once in a blue moon I find myself letting the clutch out when I first get in the Stang with no gas and stalling her out. That's always cool at the drive-thru. I'm usually like "Yeah, don't worry about it, this is my mommies car. Step off." lol