The Brother DCP 135C printer uses the LC970 ink cartridges. This brand has individual cartridges for each of the four ink shades, but the refill instructions are the same for each cartridge. The cartridges use a mylar bag to contain the ink in each cartridge. You have to create your own fill hole for these cartridges. This poses the chance that the cartridges may leak after refilling, so do so at your own risk.
Cut through the cartridge's wraparound seal using a penknife, making the cut along the line where the two halves of the cartridge meet.
Open the cartridge, grabbing and separating the two halves at the nozzle end.
Fill your syringe bottle with the correct colour ink for the cartridge. You need 20 millilitres for black ink and 10ml for any of the three colour inks.
Pierce the bag within the cartridge at the top using the syringe needle, and slowly inject the ink into the cartridge.
Seal the pierced fill hole with tape. You shouldn't need a lot of tape, but use enough to ensure that the hole is completely sealed off with no leaks.
Reconnect the two halves of the cartridge casing, and seal them with more tape at the spot of the original seal.
Repeat the steps for any other cartridges within the printer. Wash out the ink residue from the syringe bottle, and dry it completely before moving on to another shade of ink.
Install all refilled cartridges back into the printer, and run cleaning cycles and/or a test page to test the cartridges.