Two more tips:
New strings tend to stretch as they settle in to the instrument, causing the instrument to go flat. This means you will have to tune the harpejji (or any stringed instrument) more frequently with a new set of strings. There are ways of minimizing this
When changing strings is to "stretch" the strings before locking them in place in the tuner. When installing strings, I pass the strings through the tuning mechanism and then pull on them with a needle nosed plier.
Strings on tapping instruments are generally under less tension than other strung instruments, so you don't need to pull with all of your might. I pull the strings tight enough so that they are a pitch or 2 above the desired tension of the string, and slightly loosen tension back to the normal pitch (or just a hair sharp) before tightening the string with the clamp. This "pre-stretches" the string slightly, and reduces the amount of additional stretch that will occur bedfore the strings settle in.
Also, I use the curved handle end of the pliers to slightly depress the strings in front of the saddle. You don't need to depress too hard. This helps ensure a good contact between the string and the saddle, and pre-adjusts an angle in the string that is likely to move slightly as the string settles in.