The instructions below are what came with mine. There are two plugs on each cart - top clear coloured and a coloured plug for each ink colour. My understanding is that you remove the clear plug and leave the coloured plug in, then when finished filling you put the clear plug back and remove the coloured one just before installing in the printer. This presumably created a negative pressure in the cartridge to help with the ink flow.
From memory I filled mine to where I have marked with a red line in the attached photo. That is a lot lesss than the 80% they mention in the document below but I wanted to keep the level below the coloured "air hole" vent. There is a plastic internal rib there which helps to judge the level. The Chinese carts work fine but some inclusing myself have had them leak a bit from time to time. Now I use the OEM carts with a set of ARC chips on them. I weigh them when filling and they seem to work well.