There are a few steps to brewing potions so if you're unsure how to get started then you'll need to read our brewing guide. To craft the slow falling potion you will need:

A Brewing Stand

Blaze Powder

A Water Bottle

Nether Wart

Phantom Membrane

First, add the blaze powder to the top left and wait for it to melt and power the brewing stand. Next, you'll need to place the water bottle in the bottom left slot, just under the yellow line your blaze powder created. Now add the nether wart to the top slot, just to the left of the down arrow. Once this has dissolved you will need to add the phantom membrane to the same place. This will produce a regular Potion of Slow Falling that reduces falling speed and prevents fall damage for 90 seconds.