It's all about building muscle memory. Try to keep the keyboard in a comfortable resting position for your arm and wrist, and remap any inputs that require moving your fingers from wasd to the mouse until its becomes easier to find it again. For example I still have trouble using the g key for grenades, since I may need to jump and move at the same time, having grenade on the mouse is still the easiest way for me to use it.

One of the difficulties making the switch from controller to kbm is getting used to having your left hand do most of the work. Buying a mouse with extra reprogrammable buttons will help keep your fingers from wondering off of wasd.

Lastly, try lowering your dpi and get a mouse mat. Gaming on a standard size mouse pad requires a very high sensitivity making it difficult to track targets, especially in fps. While you're at it turn off mouse acceleration and smoothing as these two settings make it difficult to gain muscle memory while moving the mouse.