My college project was to design a normal drone however the department changed it and wants us to control the drone with a smartphone. It'd be of great help if someone could guide me how to make it. Or if you have a ready made one. Please help me.
My final year is on line. Thanks in advance.

Bro , use a telemetry to control the drone via phone with a good range of about nearly 2 km.Or u could use wifi receiver etc. It's the same way we use normal quads.

I'm totally new to all this. Can you explain me more briefly. Where do I start? Do I start of building a basic Rc drone and then add telemetry. And also what flight controller do I use?

If you know about arduino then i would suggest you to use bluetooth module HC-05  but limitation is of range.

you could consider adding an android soc and touch screen to a standard rc transmitter

thus making a transmitter qualify as a phone for the purposes of the competition

or

https://www.roboremo.com/uploads/2/4/5/7/24571986/esp8266-wifi-car-lua-code.zip

check out the above code as a starting point

As taksh said , you could use Bluetooth module if you are good at arduino.
Yes , you should build a drone and add wireless telemetry to it. In terms of autopilot , you could go for an apm clone if you want to cut budget or go for a Pixhawk or dji naza

You could use 3dr tower app for the control of drone. But, it won't have the joystick control ,. Only the waypoint navigation if I remember correctly. There are various other apps which offer joystick control.