If you want to mail a letter, you must apply a postage stamp to the envelope before sending it. If you have a 2007 First Class stamp, you can still use it, but its value depends on which type of 2007 stamp you have. Two First Class stamps were released in 2007--the Forever stamp and the 41-cent stamp. While you can use the Forever stamp anytime despite price changes, you must add extra stamps to use a 2007 First Class 41-cent stamp to make up for the price difference. The current price of a First Class stamp is 44 cents.
Determine how many stamps you need to apply to your envelope. If you are using the Forever stamp, you only need to apply one stamp to your envelope. If you are using a 2007 First Class stamp, which is worth 41 cents, you also need three 1-cent stamps so your envelope will have enough postage.
Remove the 2007 First Class stamp from the booklet. Be careful to hold the stamp by the corner so you don't allow the adhesive to stick to anything besides the envelope.
Apply the stamp to the top, right corner of the envelope. Use your finger to press the stamp down firmly on the envelope so it won't come off. If you need to add 1-cent stamps, go ahead and add these now.