If you want to use your won letterhead (or if you want ot email accounts and receipts) then you will need to put your letterhead in Personal Preferences / Alter / Letterhead. This is what you do.
If you already have a pritned letterhead. You only want the letterhead part in PP so scan your account / open it with eg preview then take a screen shot of just the letterhead (Hold down Shift / option / 4). You will get a different cursor with a cross hair on it. Drag this to select just your letterhead. This should give you a ‘Screen shot’ on your desktop. Open this and copy and paste into the PP / Alter / Letterhead box
If you make up your own letterhead depending on the program you used wither copy and paste it into PP / Alter / Letterhead or use the screen shot option as above.