Use
Press ` (Grave Key, next to "M" on standard European QWERTY keyboards, below the "ESC" key on standard American or Australian QWERTY keyboards, next to the "Z" on standard Apple keyboards) at any time to put your inventory into the BoH and pull other stuff out. You can add another column of storage to the BoH by crafting it with an enderpearl, a dark iron ingot, and a bit of leather as seen in the picture below
.
The best place to put your Bag of Holding depends on how many columns it stores! A new BoH should go in the 4th column from the right; this way the location of items won't change as you use `.
Specifically, you'll want to place the bag such that the number of inventory columns it stores is a multiple of the number of columns to its right.
Fair warning though, any time the BoH is upgraded, previous items in the bag will be deleted when upgraded. Make sure to take out everything before upgrading to keep your items safe.
Crafting[]