In any program . . .
Method 1: Use the Character Map. It's accessed from Start > All Programs > Accessories > System Tools > Character Map.
(Useful hint: So that you don't have to go through that lot every time you want to access the character map, go to Start > All Programs > Accessories > System Tools, then RIGHT-click on Character Map and select 'Pin to Taskbar'. Then in future you can simply click on its icon at the foot of your screen to access it). The actual method for using the character map itself is fairly intuitive.
Method 2: Use ALT codes. You need to have the Number Lock on your keyboard turned on. (It's usually on by default anyway with most computers). Then you use the numeric keypad on your keyboard (NOT the numbers across the top of it) to enter the appropriate numbers indicated in this list, while holding the ALT key down:
http://www.tedmontgomery.com/tutorial/ALTchrc.html
(I keep a printed version of that page by my computer). If you're using a laptop without a numeric keypad it's not so easy though. (You can still do it but you need to hold both the ALT and Fn keys down and use the coloured numbers which appear above some letters on your keypad).
Method 3: Use the Microsoft Word shortcuts (which are mimicked in most other word processing programs, including OpenOffice writer), which are explained here:
https://support.office.com/en-gb/article/Keyboard-shortcuts-for-international-characters-108fa0c1-fb8e-4aae-9db1-d60407d13c35