Windows, Using Roman Alphabets
(Note that the various versions of Windows vary in details and vocabulary but have somewhat similar methods.)

• Change/add Keyboard
• International English Keyboard (the easiest for a combination of many Western European accent marks)
• Alt Key Codes
• Character Map or PopChar
• Other European, Right-Left, and Asian Keyboards

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• Change or add Keyboards
This site has info for Win 10. The first steps: click the windows icon at bottom left,then Settings, Time and Language, Preferred languages. After you have multiple keyboards, to switch between them, press the Windows key + Space or click the Language indicator on the task bar near the time/date on the lower right.

Keyboard layouts for many languages are available. However, they usually do not follow the "QWERTY" layout and some characters, especially punctuation, will not correspond to those printed on your keys. French uses "AZERTY" with letters a/q and z/w interchanged compared to the US keyboard.

for layout charts for several languages.
Once the language keyboards are installed, using alt+shift or a designated variation rotates through them (adjustable by changes after pressing the "Key Settings" button. Click the taskbar language icon to switch to another language.


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• The US-International English keyboard layout is installed as above and has these features:
• It uses the following intuitive methods which work with most (or all) Windows applications, while keeping the familiar QWERTY keyboard. 
• Press one of the five modifier keys `  '  "  ~  ^ , then the letter to be modified. ( ' then a = á,   " then u = ü, ' then c = ç, etc. )  
• Press the right alt key  + another key. Examples:
  right alt + ,  = ç  (or  ' + c) 
              + / = ¿ 
              + 1 = ¡ 
              + c = © 
    + 5 = Euro currency symbol       for a complete chart and printer-friendly summary.
• Note that this maintains the "qwerty" layout. However, the modifier keys `  '  "  ~  ^  must sometimes be followed by pressing the space bar when they are actually intended. The system can accept words requiring an apostrophe, such as it's, without the space bar.   
• If you are having punctuation key troubles such as your computer's quote and apostrophe key behaving strangely, the cause may be that your keyboard was accidentally set to the International English Keyboard. The solution is to reset it to a standard English or United States keyboard. 

• Alt Key Codes (also called Alt number or altnum)
  
• Those who use only a few characters, but often and in many applications, may prefer to keep the default United States English keyboard and memorize a few Alt key codes. This method uses the Alt key plus a four digit number entered via the numeric keypad, with Num Lock on. The modified letter appears after the keys are pressed and released. For touch-typers, the disadvantage is having to move their hands to the number pad on the right of the keyboard. Examples follow. Anecdotal evidence suggests that this works in most programs, even in email and filenames. Note that some laptops have a "hidden" numeric keypad.

• ALT + 0224 = à        ALT + 0225 = á
• ALT + 0232 = è        ALT + 0233 = é         ALT + 0200 = È
• ALT + 0242 = ò        ALT + 0243 = ó         ALT + 0241 = ñ       for complete lists (including Esperanto).
 
• Character Map
• This is useful for the infrequent accent or symbol. Use the search box near the lower left to reach it. After choosing a font, double click the desired character(s). After clicking Copy, return to your document and paste. Some fonts (such as Symbol, Webding, and Wingding) provide Greek, icons, arrows and symbols. This system may fail if a combination of keystrokes, as seen in the lower right of the window, has been reserved for another application.
  
• PopChar is an improvement over the Character Map. When you need a special character, click the "P" box in the taskbar to display a table of characters. Select the desired character and it instantly appears in your document. HTML symbols are also available.

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1. Press CTRL and one of the punctuation keys ` ' , ~ : ^ that most closely resembles the accent needed. 
2. Release the two keys pressed in Step 1.
3. Press the letter to be modified and the accented character will appear. 
Thus Ctrl+: with u gives ü and Ctrl+, with C gives Ç. 
The following chart came from MS Word help/index/characters/inserting/type international. Newer versions have slightly different wording such as help/index/character, then click on Insert an international character by using a shortcut key. Note that some keys such as & require a shift key also.

Math and other Symbols

Word 2010 has excellent features which one can reach by choosing the Insert Tab and then going to the symbols section of the "ribbon". Clicking on "Equation" provides about ten built-in equations including the area of a circle and the quadratic equation. Clicking on the Π above "Equation" opens a ribbon of math terms similar to those in the 2003/2007 math equation editor. The drop-down arrows lead to many more terms. Clicking on "Symbol" results in a variety of those, many of which can be pasted into other software.Click here for a chart of math and music symbols using the alt plus 4 digit codes..

Some Word 2010 features are similar to Word 2003/2007 where many non-English characters, as well as some icons are found in the Insert menu, then select Symbol and More Symbols. Within this Symbol Window, the font Lucida Sans Unicode provides many accented and other characters for numerous languages.The Wingding font has many other characters. The Character Map (see above) also has these fonts.

Word's Equation Editor is available on the Insert menu in the Symbol section. Purchasing a more convenient math editor such as Math Type is advised for those who write equations often.

More Math tips for MS Word:
*The Symbol font (in the Home tab) also  has many math characters. For example, after changing to the Symbol font, press the tilde key at upper left, without the shift key. You should see a bar. Then type another key such as s; the result for s is a lower case Greek sigma below the bar. If you need an x or X or other letter under the bar, change the font back to a text font such as Times Roman, and then type x or X.
*For superscripts, such as xⁿ: type x, then press and hold Ctrl, Shift, =. Then type n. To go back to normal letters, press and hold Ctrl, Shift, =. Subscripts are similar but omit the Shift key.

for complete lists

In a software application such as Dreamweaver, one can use the International English keyboard. Also, Dreamweaver has Insert - special characters in the menu bar. The shareware application NoteTab provides another method as well as other useful features.