You make custom banner designs by using a loom block to combine a base banner, dye colors, and optional pattern templates.
Use the Loom Block
While crafting tables used to handle banners in older versions, the loom block introduced in version 1.14 is now the official method. The loom cuts the dye cost from up to six units down to just one unit per layer. You place your base banner in the left slot, your dye in the middle, and select from the pattern list on the right to make custom banner designs easily.

Add Special Patterns
Certain advanced designs require a physical banner pattern item, such as the Creeper Charge, Skull Charge, Thing (Mojang logo), Snout, Flow, or Guster patterns. You place these paper sheets in the bottom slot of the loom. The generator includes all of these special banner patterns and recipes so you can preview them before crafting.