One of the best ways to increase the security of your Chaindeck is to utilize False Markings.
What are False Markings?
After you've completed writing your Secret Phrase to the sides of your deck you'll shuffle the cards. Even after a long and random shuffle you'll notice there will be a lot of empty spaces available on the sides of the deck. False Markings are the random words, letters, and lines you can draw in these empty spaces in order to throw-off an attacker by creating "dead-ends".
Why use False Markings?
Anyone trying to maliciously access your Secret Phrase may try to solve your Chaindeck like a puzzle by matching lines across cards. They would try to find two cards that seem to have the same line continue over their sides. False Markings make this significantly more difficult by creating the illusion of cards being found in the right arrangement when in fact they actually are not in the correct arrangement. Furthermore, having more lines and ink on the sides of the cards make the odds of creating incorrect matches more likely, making it even more difficult to attack your Chaindeck.
Important Considerations
Chaindeck delays access
First and most importantly, Chaindeck is a device to delay access to your Secret Phrase. Any Chaindeck can eventually be deciphered.
Don't write over your Secret Phrase
When placing False Markings do not draw over existing markings. Doing so will make reading the real Secret Phrase more difficult.
Use the same pen and style
Make sure your False Markings look just like your Secret Phrase. Use the same pen. Make the letter and symbols the same size as the Secret Phrase.
Check your Secret Phrase afterwards
Once you're done writing the False Markings, it is best practice to use your PIN and chain your deck again to ensure your entire Secret Phrase is still legible. You can bold any parts that may have become harder to read.
Tips
Leave room for False Markings
When you're writing your Secret Phrase make sure that it is clear and legible. However, making it as small as possible will allow you more room to place False Markings.
Rearrange the cards for bigger space
When you're writing False Markings you can manually rearrange the cards to open more empty space for longer words. As long as you've shuffled the cards once after writing your Secret Phrase, it is alright to change the arrangement of the cards to make room for the False Markings.
Reshuffle and add more
You can do more than one round of adding False Markings. Reshuffling the cards will typically open up more space free from markings. You can fill those new open areas with false letters and words, and repeat as you wish.
Use a False PIN
This is an advanced method that'll take more time to increase security. Instead of shuffling the cards, you can pick a False PIN and chain the cards using it. You could even write an entire False Secret Phrase that goes with the False PIN.
Get your Chaindeck and start utilizing false markings.