Using an index card is a simple technique to help you focus during the day. This is not a replacement for your task application or calendar but an addition. The point is to help you be less distracted and keep your planned tasks front and center while you work.
Review your task application and calendar every morning (or the night before) and determine what needs to be done that day. Write the main things you want to accomplish on your index card.
Use your index card as you work during the day. As things come up, jot them down. You can even take brief notes from meetings or log things that happened.
When you shut down for the day, open your task application and check off any completed tasks at the end of your day. Add any new tasks. Open your notes application and add any notes from your index card. This is an opportunity to clarify those notes and make them meaningful. Skip anything that does not provide future value.
You can use some standard 3x5 index cards. Or find some vertical To-Do index cards that work great for this.
You can also find yourself a nice business card holder that can be used to hold your cards.
A search on Amazon returns all kinds of options, from cheap solutions to more fancy, custom solutions.
If you want to go all out, Analog by Ugmonk has a high-quality card system that you can purchase. The Analog kit includes a walnut or maple card box with a magnetic lid to secure your other cards, and it also comes with Task cards for Today, Next, and Someday.