Be forewarned that attempting to make a finished non-VR game support VR has a high probability of leading to a poor VR experience. That’s not to say it can’t be done but for it to be done correctly will take a significant amount of work. Most of this is all dependent on your existing gameplay and game design. Without knowing anything about it, it’s hard to truly say what the work effort is. I would suggest starting with the VR best practices guide in the wiki.
If you have a 2D UI, you will need to create an all new 3D UI for VR (not fixed on player camera). If you made a realistic game but wasn’t careful to ensure every asset was the correct scale, you’re going to have your work cut out for you. This is especially a concern if you made a third-person game where its common in the industry to make rooms larger than they really are to accommodate the camera distance and height. One of the most important things is going to be performance. Just because your game runs fine without an HMD doesn’t mean it can handle being ran in VR mode at the minimum 90FPS @ 2160x1200. Even if you can hit that framerate at that resolution on your monitor doesn’t mean you will in VR mode due to the roughly 40% performance loss from rendering in stereo.
Sorry to be mainly negative but trying to cram in last minute VR support into a finished game without any experience in the platform is a high-risk move, not just for yourself but other developers releasing a VR supported title because poor VR games and experiences this early in the technologies lifecycle can have a huge negative impact to consumer perception.
My recommendation, hold off on adding full VR support and start smaller with an experimental VR option. Maybe a mini-game or smaller subset of your game and clearly communicate that the support is still experimental and as you learn what makes a good VR experience, continue to expand upon that. In the meantime, Epic has posted some great Twitch streams on getting started with VR. The Oculus forums are a great source of VR related advice - most is either generic or specific to Unity but the info is still relevant. Also, if you dont have access to an HMD, get one and play as many VR titles as you can to figure out what does and doesnt work.
Good luck on your game!