I actually wonder why nobody mentions the docking mode for KSP. You should switch to it if you are about 100-50m away from your target and your relative velocity is about 0. It's the button in the lower left corner between the map and the staging control or you can hit the "delete"-key on your keyboard. When you are in docking mode, you can switch between rotation-movement and linear-movement by pressing space. First, you should go to rotation-mode and try to line up your docking ports parallel to each other... you spin your craft with WASD and QE (roll) while in rotation-mode. Then, you switch to linear-control by hitting space again (all in docking mode ofc) and move your non-rotating ship towards the docking port of your target. While in linear-control, use SHIFT and CTRL to move it upwards or downwards, W and S to move forward and backward, A and D to move left and right. If your angle isn't right, just switch back to rotation-control (hit space) and turn your craft around its axis so it's lined up for the "linear-approach". All these maneuvers are only done with RCS-thrusters. Also, while in linear-control, SAS is activated automatically to avoid spinning of the craft. If something is unclear, feel free to ask