There are three main styles of refillable pens. There are non-retractable pens. There are retractable pens that extend and retract with a push button and there are retractable pens that extend and retract by twisting the base of the pen. Before you do anything check to make sure that they refillable pen and the disposable pen have tips that are the same diameter. Otherwise this hack will not work. If you have a non-retractable pen, you may be able to replace by simply putting in the new reservoir from your disposable pen. You may have to trim off the spring tabs if it doesn’t fit. If you are refilling a pen that extends and retracts by twisting the base, cut the ink reservoir from the disposable pen to the same length as the original tube. Then just fit the plastic end cap on the new reservoir and reassemble the pen. Again you may have to trim the spring tabs if it doesn’t fit back together. If you are refilling a pushbutton pen, first remove the plastic end cap from the ink reservoir and cut the ink reservoir from the disposable pen to the same length as the original assembly minus the length of the end cap. When reassembling the pen simply place the end cap back into the pen where it would normally sit and then place the new ink reservoir on top of it. This will allow the normal push button action to work. This hack should work for most medium tip ball point pens. But keep in mind that there are slight variations between brands and styles so it might not work 100% of the time.