On my CISS system there are two plugs for each bottle. One is the filler and the other is the air vent. I just remove the filler plug, fill the syringe and inject the ink into the bottle. I quickly wash the syringe between each colour. I get my ink from Disk Depot at £7 for 6x100ml bottles.click here
Don't believe the rubbish written about CISS systems (usually by people that have never owned one); the colours are vibrant, there is no difference between colour prints from Epson inks or from the CISS inks and the blacks are definitely better than than the Epson inks that I used to use. If you do a lot of printing(or even if you do a small amount) a CISS system will save you shed loads and it is worth buying a compatible printer for the inks (usually Epson or Canon.
ps..the instructions that I received with the system were odd to say the least but a little thought and head scratching and it was easy to install (10 mins.)