Yes, you first need to buy the shifters themselves, then check if your existing steering can fit those paddles. There is the 335i shifters and those from the M3. The M3 is the more conventional type of shifters where one is upshift and one is downshift whilst the 335i ones are different.
Then once you are done, you need to work on the wiring harness to connect from the steering wheel to your gearshifters.
There are two ways, either you get the cable harnesses or you can splice the wires. The harness route is the safest route.
Becareful with some of the instructions you find online especially when removal of the steering wheel as you need to make sure you don't set off the airbag.
I have read through a lot of the DIY methods and personally, I wouldn't dare go through that route even though I am an electronics engineer and would rather get a plug and play system instead.