The answer is online which I've copied below -
http://files.support.epson.com/htmldocs/nx300_/nx300_ug/wwhelp/wwhimpl/js/html/wwhelp.htm
Replace Ink Cartridges : Remove and Install Cartridges
---- Remove and Install Cartridges
Be sure you have your replacement cartridges handy before you begin. You must install new cartridges immediately after removing the old ones.
Caution:
Do not open ink cartridge packages until you are ready to install the ink. Cartridges are vacuum packed to maintain reliability.1.
Turn on the NX300 Series.
2.
If a cartridge is expended, you see a message on the NX300 Series display screen. The message on the screen indicates which color to replace. Press OK.
If you're replacing a cartridge before it's expended, press the Copy/Fax button, press the Menu button, press or to select Maintenance, and press OK. Then select Change InkCartridge and press OK.
3.
Open the scanner.
Caution:
Do not lift up the scanner unit when the document cover is open.
4.
Open the cartridge cover.
5.
Squeeze the tab on the cartridge and lift the cartridge straight up to remove it. Dispose of it carefully. Do not take the used cartridge apart or try to refill it.
Warning:
If ink gets on your hands, wash them thoroughly with soap and water. If ink gets into your eyes, flush them immediately with water. Keep ink cartridges out of the reach of children.
6.
Before opening the new cartridge package, shake it four or five times. (Do not shake a cartridge after opening the package; ink may leak.)
7.
Remove the cartridge from the package, but be careful not to touch the green chip on the cartridge.
8.
Remove the yellow tape from the bottom of the cartridge.
Caution:
Do not remove any other labels or seals, or ink will leak.
9.
Insert the new cartridge into the holder and push it down until it clicks into place.
10.
Once you replace all the necessary cartridges, close the cartridge cover and push it down until it clicks into place.
11.
Close the scanner.
12.
Press OK to begin charging the ink. This takes about 2 minutes.
If you see an ink replacement message on the display screen, press OK and press down all the cartridges securely. When you see a completion message on the display screen, ink charging is finished.
Caution:
Never turn off the NX300 Series while ink is charging or you'll waste ink.
If you remove an ink cartridge for later use, protect the ink supply area from dirt and dust and store it in the same environment as the NX300 Series with the label facing upward. Note that there is a valve in the ink supply port, making covers or plugs unnecessary, but care is needed to prevent the ink from staining items that the cartridge touches. Do not touch the ink cartridge ink supply port or surrounding area, and do not store the cartridge upside down.
If you remove a low or expended ink cartridge, you cannot reinstall and use the cartridge.
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See this link for installing the ink cartridges:
http://files.support.epson.com/pdf/nx100_/nx100_bb.pdf
you first press the tool button and then the right button and finally the stop button.
How to scan with Epsoon Stylus NX300
An Epson Stylus nx300 is not very different from any other fax machine. Connect the fax machine, then insert the document, dial the fax number, then press send. Specific instructions can be found in the user manual on page 14.
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