Some word processors aren't able to visualize all the characters, which means you need to memorize the keyboard shortcut to insert a character. Using BabelMap, you'll be able to copy characters to your clipboard and paste them into your word processor, as long as it's compatible with Unicode.
The way the program works is very easy. Once it's open, you just need to look through a chart full of characters and find what you need. You can also refine your search to certain blocks of characters in order to find what you're looking for more easily.
For example, if you need a specific mathematical symbol, you just have to go to the option 'Select Unicode Block', then select a math block. There you'll be able to browse through everything available and find the symbol you're looking for.