Putting on a sack and ashes is a very big Tikun
The best time to do so is during a fast, Tikun Chatzot and when doing Vidoy

-The best type of sack is one made from goat hairs or any animal hairs, as this is the type the Ari brings down.
-In the end any type of sack will work.
-Go to the local garment shop, where they sell pieces of materials. Most have a roll of sack. Buy a piece of the materiel that extends length to length with a calculated hole for the head (image 1). Cut a hole for the head in the middle. Now cut off a piece to use for a belt (image 2). Now round off a two corners so there will be no problem of Tzizit (image 3).

-It is best to wear the sack on your body, put it under your undershirt. If you refuse to do that then wear it over your clothes, it is still a big Tikun.
-Note you can easily wear it in a way that no one will notice it. You can go to work and run all over the office without anyone noticing it.

As you are putting on the sack do the following Kavanot.
Everything works without Kavanot, but the more Kavanot the more effective it can be.

(Taharat Hakodesh)