Find the numeric code that corresponds to symbol you want to type in.
You'll see all Alt codes listed below.
Enable Num Lock.
Usually Num Lock is a button located on keyboard's right side (on numeric Keypad). It "unlocks" the numeric pad. When you'll enable it, the green Num Lock indicator will shine green.
Press "Alt" (button).
You have to press down and hold Alt key button on your keyboard.
Type the Alt code in (using Keypad keys).
Do this while holding Alt key pressed. The code needs to be entered on the Numeric keypad (right key pad on usual keyboards). It's a key pad on part of your keyboard. Look at the image to see how it looks. Enter there the numeric Alt code of symbol you want to make.
If you did everything right then the symbol should have been inserted after you've released the Alt button.
If it didn't work out after you typed the code while holding right Alt key - try holding down the left one.
If you're really interested in alt codes - read more info on them in Short details on Alt codes.