This rocket will go to Duna and back to Kerbin (overkill)
The lander is on top of the rocket, just activate the decoupler to detach it once in orbit.
Once in Low Orbit around Kerbin, you will have 5500 DeltaV to go to Duna + 5000 DeltaV on the Lander.
Don't forget to add some Kerbals inside the rocket.
The engine has a low ISP, so it takes longer to get to Duna, about 5 minutes, and another 3-4 minutes to Orbit Duna, but the upside is you're going to get a lot of Delta V. Be careful with the lander, the engine is not really powerful so you will have to turn on the motor quickly to slow down.
KSP Builds - Duna V12