The OpenDocument file (OpenOffice.org 2.0+) that was used to generate the screenshot is at
http://www.simos.info/files/CopticSample-CombiningDiacritics.odtI know how to type these letters in Linux (Ubuntu, Fedora, etc) when using the GNOME graphical environment.
Other operating systems should have a similar procedure.
In GTK+ applications on Linux you can type Coptic using the codepoint.
For example, for lowercase alfa,
1. keep pressed Ctrl-Shift
2. type 2C81
3. release the Ctrl-Shift keys.
Of course, the font needs to support Coptic so that the character is shown. In Gedit or OpenOffice.org you need to select a suitable font. I think in newer versions of Linux a capable Coptic font will be autoselected. In OpenOffice.org you need to hit the spacebar to get the character shown, as a step 3.
To add combining marks, follow the list shown at
http://www.travelphrases.info/gallery/Test_Coptic.htmlwhich shows the acceptable combining marks for Coptic and their Unicode codepoint value.
For the grave combining mark,
1. keep pressed Ctrl-Shift
2. type 300
For the combining mark to work, you must place the cursor just ahead of the coptic letter (alfa). The combining mark will automatically attach to the letter that is position on its left. You can do the same trick even with english, as a way to add combining marks.
It is evident that the fonts need special editing so that the combining marks are centered well on the letters.
I do not know which Coptic letters are permitted to have combining marks. In Greek, letters like Beta do not get a combining mark. What's the situation in Coptic?