For those readers who want to make tilde accents over characters instead of the NORMAL tilde sign (~) requested by Aborted, here is some possibly useful info:
OptionN applies a
~ to the next
n that you type (given that you release them first). In the event that your keyboard/keyboard settings do not afford you the character, more modified character options are available in the keyboard preferences option available from the toolbar in the upper right corner.
select "Latin" from the option on the left, then double click on your desired character to insert at your cursor.
Another option is the Accent Menu you get when you hold down the key for the base letter, as described by Apple here