To change the bushings you first remove the big nut in center of the roadside part of the truck using a skate tool or ⅜ socket. This will give you access to taking off the washer, bushings and hangar beneath. Do so and then place your new bushings in the place of the old ones. A harder boardside bushing compared to roadside is ideal but not necessary. If you do have 2 different hardness bushings for the same truck, make sure the harder one is on the boardside. If you have a different overall hardness of bushings between your 2 trucks, the harder ones go in the rear. If there is any confusion left after this, think of the order of bushing hardness, in terms of softest to hardest as, front/roadside, front/boardside, rear/roadside, rear/boardside.
After the bushings have been exchanged, tighten down the kingpin nut to just slightly touching the roadside washer and then tighten around another half a turn. For different manufacturers this amount can vary, but not by much. If the trucks still don’t feel right, you still need new bushings or need to get used to them. The same bushings will feel different based on a few things so most bushing guides are not super accurate.