The most common way is to steal the Fishing Rod. A less common but faster method in some situations involves riding a horse Lon Lon Ranch without having previously obtained a sword.
Both methods typically put a Deku Stick on your B button. With a stick on B, that stick can be turned into a Bottle with the Bottle Duplication glitch as long as you have 0 sticks. If you have more than 0 sticks, you will pull one out instead, which can crash the game as an adult on certain versions, so make sure you get rid of any sticks as a child first.
However if you steal the Fishing Rod, you can go straight to a bottle on B with the shield swipe strategy.
See Steal the Fishing Rod for more information.
Due to several of the Forest Escapes, combined with Swordless Door of Time Skip, it is possible to avoid equipping a sword directly through the equipment screen interface by the time you become an adult. This results in a rather interesting oddity with Ingo's horse riding minigame game at Lon Lon Ranch. In order to execute it, follow these steps:
One thing to note about this method, after you have obtained the stick (and subsequently, the bottle) on B, if at any point in the game you savewarp, deathwarp, or fail a timed trade item sequence, your B button will now be blank again, and you would have to repeat step 5 above. So if you still have future Reverse Bottle Adventure or Bottle Adventure things to do, keep this in mind.
A simple way to avoid the above problem is by duping a bottle over the master sword just like a normal bottle dupe. Swing your bottle on B at something you can catch and equip the Master Sword mid-swing to dupe a bottle over it. You will now have Stick on B rather than a blank B when you savewarp, die, fail a timed trade item sequence, or get caught by guards.
So once you have the Deku Stick or Fishing Rod on B, it is time to turn it into a bottle on B. To do this, simply perform a Bottle Dupe over your B button item. An important note, if you are playing on any N64 version, the game will freeze if you pull out a deku stick as an Adult. However, on the GCN and VC versions, you will instead weild a glitchy deku stick. To change the stick into a bottle you must get rid of all of your Deku Sticks. If you're playing an N64 version as an Adult, you must get rid of all of your sticks as a Child first. Since putting a bottle on B also activates Reverse Bottle Adventure, it's best if the real bottle you are using is equipped it on C-Right, or make sure your first C-Right item of Reverse Bottle Adventure is actually on C-Right.
You can quickly get a bottle on your B button by duplicating a bottle over your sword. This is the fastest and most flexible method to get a bottle on your B button and pretty much obsoletes the other methods, but they are still included for documentation.
If done correctly, you should have a bottle duplicated over your sword. This trick can be done in many places, but the most common areas are Hyrule Field and Lake Hylia.
While shielding damage, there are a few frames where you can draw the sword by pressing B. If this is interrupted by another damage source, you can activate quickdraw. If you do this with a bottle in your hand, you can just Shield Swipe to duplicate a bottle over your sword.
While time stop is active, you will not put items away automatically when entering deep water. This lets you keep the bottle in your hand while in the water. Press B when exiting the water to change the last item slot used the B Button. Then just shield swipe something and you get bottle on B.
If a text box is up while you are swimming, your items will not be put away automatically. You can press B while swimming to change your last item slot to the B Button and then Shield Swipe to place a bottle on your B Button. You can use the text from a hatching egg, a gossip stone, or a gold skulltula token, as shown in the video below.