Tmcase is right, if you are asking about mixing your own juice then head over to the mixing sub-forum linked in tm's post and start reading.
If you are asking about filling your own cartridges (carts), or cartomizers (cartos), or clearomizers (clearos) with premixed juice from any of the numerous vendors, that is a different question and it would be easier if we knew which equipment you are using. (Carts are fairly rare now days with cartos being much more popular.) Of course you can fill your carts, cartos, and clearos with your own mixes too, but it is real handy to have a dripping atomizer on hand for testing your mixes before you commit to filling up your equipment.