Psalm 91 also appears in many versions of the ancient Bedtime Shema (found in any Orthodox prayer book).
According to the Chida, Psalm 90 protects against lions while Psalm 91 protects against an "evil spirit" (ruach ra).
(Please see full list here: the-full-list-of-segulotuses-for-all-of-tehillimpsalms.html)
Rosh Yeshivah of Slabodka Rav Dov Landau has also called for everyone to say Psalm 91.
If you understand Hebrew very well, you can listen to Rav Mazuz here:
https://www.torahanytime.com/#/lectures?v=258954
In short, the Gemara Shavuot labels Psalm 91 as the Psalm of "pega'im" — harmful events.
The rav explains (also in the name of the Chazon Ish) how this means it's a Psalm against pigu'im — terror attacks.
At minute 1:10, the rav gives the example of a non-Jewish military officer who refused to start a day of battle until he and all his soldiers said Psalm 91.
As a result, Rav Mazuz states, for the duration of 5 or 6 years of war, they never suffered any injury — not even a scratch.
The rav insists (and I believe him!) this is the best protection against terror attacks.
This is how you say it:
(The rav notes it's best to say it right after the Shacharit morning prayers. If not then, then before the Mincha afternoon prayers.)
First, you say the last verse (verse 17) from Psalm 90:
"And may the pleasantness of the Lord our God be upon us, and the work of our hands establish for us, and the work of our hands establish it."
Then you recite all of Psalm 91.
May Hashem please return all Yidden everywhere from captivity, heal all the ill and wounded, and immediately bring Mashiach and the Geula with Compassion.