To find vacuum leaks you can use starter fluid spray. Do a quick google and you can see how its done. Also just check all the hoses and see if any of them are brittle and replace as needed.
Also check the IAC value this thing gets really dirty and can cause idle issues. You can pull it off and clean it with throttle body cleaner. Again you can youtube this for your car and see how its done. You could also just replace this part if its really bad I dont think the cost is that much.
2002 Ford Mustang GT, 5 Speed, EiBach/Maximum Motorsports Sway Bars, Coil Over Springs, Caster Plates, Panhard Bar, Sub Frame Bar, Strut Tower Brace, KONI Rear Shocks. Pioneer Stereo, MGW Short Throw Shifter, RatFink Handle, interior led lights & red under dash. Chrome Pedals. Cobra Hood. Led reverse lights. Tint 5% limo in the back. 50% in the fronts. AirRaid CAI, Throttle Body Spacer Plate, MDK Coil Over Packs, Borla S Type Cat Back System, Bullitt Wheels, Hankook 285/265 Tires