There are two ways to refill your clearomizer or atomizer depending on the model you use.
Most clearomizer and atomizers that are available on the market today are bottom filled, which means that they are filled with e-juice from its bottom. This is made possible by, first remove the clearomizer from your battery/mod, then unscrew the bottom of the clearomizer in order to access the part that holds the actual e-juice. See the video below for examples.
Since the beginning of 2016 the trend has been towards the majority of the clearomizers and atomizers reaches the market with a top filling system. The benefits of top filling is that you wont need not remove the clearomizer from your battery/mod to fill it up again, the risk of contact with the e-juice is reduced and it is an way easier process. See the video below for examples.
Below you can watch a video showing how to refill a clearomizer with bottom filling: