If you are using something besides Blink Ink, you may need to use an eyedropper for this next step.
If you are using Blink Ink, you must first shake up the ink. It settles. This is normal.
Take your newly shaken bottle of ink, untwist the top, and squirt ink into the pen body. If you are careful, you will not make a mess. If you squeeze too hard on the ink bottle, ink might leak out the top. It is not necessary to squeeze very hard.
Do not overfill the pen or it will not be fun anymore. Fill it halfway or three-quarters of the way full. Trust me.