For PC you may want to refer to moheb.de it has excellent documentation on how to use Coptic on computers. You need at least one unicode font, and then to carry on the installation. If it works well & good, if not and you want to type a document in Coptic; you may want to check copticchurch.net & download the older CS fonts from their Coptic Fonts page. However, the CS is a temporary solution for small documents. CS fonts basically tricks the computer that this is a weird looking English font, hence the receiver on the other end MUST HAVE the same exact font that you are using otherwise it will read as English.
Then, I'd recommend to use an online converter to switch whatever text written in CS fonts (e.g. CS Avva Shenouda) to unicode
http://www.copticproject.com/coptic/unicode-converterAn alternative method is to type on a website that provides an online unicode Coptic Keyboard then paste your text to your document e.g.
http://www.lexilogos.com/keyboard/coptic.htm has good support for Coptic letters, you can either type (some letter as theta/teta is typed as a quick t+h otherwise you can use the mouse to select the letter.
For mobile phones, some older models, do not support Coptic so all apps won't work, the ones that support would work well with most apps. The one that I'd highly recommend is SwiftKey Keyboard, it's the default keyboard app for many mobile phones, if it's not you can install it and make it the default. It has in-built Coptic language in its later versions. You can add the language just like adding any other language so that you don't have to switch between two different keyboard apps. It also has the added benefit of autocorrect, and text prediction. So, if you use it regularly, it will keep the words you use most and will make your life easier. To date it's working well on newer Android phones, (< 1 year). I don't think it works on iPhone at present.
