First stept is to optimize your gameplay for VR so you need to define how the player interacts with the game world and vice versa. It is however different from a traditional Display game as you have more opportunities to fine grain your VR experience while you also suffer from the limitations of control.

That said, not every game is working in VR as not every VR game is working on Display!

The second part is the easiest one, get a VR SDK either from the vendor itself or use OpenXR or SteamVR as alternatives. Integrate them with your game (or your game engine already integrates them?) and setup everything to show up properly.

Step three as the most difficult part is to optimize your game for VR. Forget about flat UI and impressive graphics, everything needs to be optimized to be rendered twice and at an average rate of 72 FPS or else your VR experience gets broken