Some like to purchase pre-sized templates that can be printed to fit into their notebooks.
There are a variety of digital downloads sold on Etsy and other platforms, or sometimes offered for free, by planner nerds who are willing to share their templates.
Printables cover a variety of needs—you can print layouts to tip or glue into your planner for just about any purpose. There are printables meant to be used on sticker paper as well. This can be a very affordable way to customize your planner and all you need, of course, is a printer.
I’ll be adding to the list below, with a focus on printables that serve different functions (as opposed to printables with varying aesthetic designs).
