⇒ All lumber is different, the more tightly packed the pores are; the less sealer you will need. New lumber usually requires less sealer than old, sun baked, dried out lumber. Containers of sealer show how many square feet it will cover in ideal situations; older decks (or fences) are going to need more.
⇒ Make sure your deck has been cleaned, and it must be completely dry.
This does not mean to make sure your deck "looks" clean, it means that you must actually clean the deck because there is dust, pollen, algae, mold spores, etc.., and if you apply sealer over a dirty deck, the sealer might fail or discolor. Brand new pressure treated pine or cedar decks have "millglaze"; wood sugers forming a barrier created from the process of cutting the lumber - which needs to be removed.
If you are not going to get a professional to prepare your deck, you can use a bucket of deck cleaner solution found at the hardware store, a scrub brush, and a garden hose.
⇒ Schedule this on a day when it will not be raining & temperatures are above 50° fahrenheit
⇒ Transparent colors may look different on new lumber Vs. old lumber; sample testing your color in an inconspicuous area is always a good idea to see how it will look. Allow your sample to dry, because it looks different when you first put it on.
⇒ You need to stir / mix your sealer - some sealers need to be stirred constantly while you are applying them.
Make sure your sealer has been shaken at the store, or you can use paint paddles to stir. If you have several gallons of the same stuff, "Box" all of your sealer together into a larger container. "Boxing" means to mix all the same product from individual containers back and forth, in order to achieve a uniform color. An easy way to do this; is to use a 5 gallon bucket to pour everything into, and then stir. You then can redistribute your sealer back into the original containers and put the lids back on until you need to use it, and you will be certain that every matches - even if you need to do touch ups later.
⇒ Do not apply your sealer in direct sunlight on a super hot sunny day - if at all possible.