In the normal method of riffle stacking you start with the four aces on top. When you do the shuffles you hold back cards with both the right and left hands, dropping the cards held by the left thumb under the cards held back by the right thumb. At each shuffle the number of cards held back by the left stays the same, but those held by the right decrease.
For example, to stack four aces for five hands of draw poker you would begin with the aces on top. Cutting the top half to the right, when you do the first shuffle you would hold back the four top cards on the left half, and the three top cards on the right. You would then drop the four left cards, and then the four right.
The next shuffle would be the same except you would hold back two cards with the right.
The next shuffle would be the same except you would hold back one card with the right.
And the final shuffle you would hold back no cards on the right.
The holding back of varying number of cards makes this method difficult to do smoothly.
An easier way
Start with three aces on the bottom, and one on the top. Cut the top half to the right and begin your shuffle by dropping at least three cards (the aces) from the left hand half. Shuffle normally, holding back four cards with the left thumb, finishing the right half shuffle and dropping the four cards on top of the right hand half. This puts four indifferent cards on top of the Ace.
Square up the deck and undercut half of the deck with your right hand to the top, keeping a break between the halves with your right thumb. Riffle off one card with your right thumb and then take the top half to the left with your left hand. Now repeat the shuffle from the paragraph below, adding four cards to the top of the right hand half.
Do the same thing twice more and you are done.
The only thing you have to watch out for is to keep a large enough block of cards in the right half on top so you don’t shuffle any cards into the already stacked aces.
A not quite as easy
Similar to the previous method, except you start with two aces on the top and two on the bottom.
In the first shuffle, drop at least two cards from the left hand half first, shuffle holding four cards back on the left and only one on the right. Drop the left four and then the right one last.
In the second shuffle hold back four from the left and none from the right.
Now undercut half of the deck with the right hand to the top and keep a break. Riffle off two cards with the right thumb adding them to the top of the bottom half. Cut the top half (at the break) to the left and repeat the first two shuffles.
End note
In the video above, I start with four aces on top. I double cut two to the bottom, and then on the first shuffle place four indifferent cards between the top two aces. The next shuffle puts four indifferent cards on top of the top two aces. Next I cut the deck to add one ace from the bottom to the top, and shuffle four indifferent cards on top of that one. Then I riffle off five cards from the bottom of the deck and do a blind running cut to move those cards to the top. I finish with a Artanis false shuffle.
On the last shuffle, if you have difficulty keeping the right hand block 16 cards held back, what I will do is make the last shuffle a Zarrow shuffle. This is a false shuffle that simply adds four cards to the top of the deck.