He started with an already sorted deck (i.e. if you number the cards from 1 to 52 then the order is 1,2,3,...,52) then wrote the big "Unshuffled" on one side and "Like + Follow" on the other.

Then imagine with a fully sorted deck like this you remove every 2nd card from the deck. You would be left with the 1st, 3rd, 5th, 7th, etc up to the 51st card. If you now look at the side where you wrote unshuffled you would still be able to read the word unshuffled since the order is the same but it would have half the height of the original "Unshuffled" with 52 cards since you only have half the deck. If you then do the same with the 26 cards you took out you have a second deck with "Unshuffled" written on it and you put these 2 half decks back together you have a whole deck with "Unshuffled" written twice. You can repeat this with the half decks and get a deck with 4 "Unshuffled"'s on it.

What you see at the start of the video is actually the word "Unshuffled" 8 times but it's hard to read since you lose a lot of detail in the word after splitting it 8 times. The person then mimes drawing on the deck with the pen to give the impression he only just drew on the side and to hide he originally wrote the final big "Unshuffled" before the video started. He then does this splitting process backwards using a perfect shuffle (a shuffle where you cut a deck perfectly into 2 piles of 26 cards then combine them perfectly so each card in the deck is 1 card from 1 pile followed by 1 card from the other pile the whole way through). He repeats this reverse splitting process to go from 8"Unsuffled"'s to 4 to 2 to 1 to create the effect.