You have choices. If you are going to leave the hard caps on, go with the condom method and the Taryn spin. It's by far the easiest (Youtube, search for leaford double)
If you would prefer a different mouthpiece, step one is to get the glued one off entirely. Hairdryer, dipping in hot water, etc. Once it is off, you can replace it with a drip tip or an empty cart (the ones with no filling that came with your atomizers are good.) If you use a cart, cut off some from the bottom otherwise it will reach the filler and crush it.
While you can still do the condom method once the end has been removed, you'd need to do it from the threaded end*. Alternatives are syringe and drip. (Youtube, DoctorVapor Care and Feeding of Cartomizers)
If you drip it, you still want to do the Taryn spin.
Always put the .... plug and bottom condom on when dripping.
Replace the condoms and let them marinate overnight for best vaping.
*I haven't tried that yet. I've been dripping and flooding. I will change my ways immediately.
ETA: Most of us seem to prefer removing the end. That's why a few suppliers are getting them with soft, removable ends (and charging more for them.) But if you use the condom method from the mouthpiece side with the soft ones, the end keeps falling off into the condom.)