I've received different messages on how confusing the public stims feature is, and I won't disagree. So I've deceided to write a small guide to understand how stims are played and how to manage your stims to get the best experience with the normal session mode.
There are two types of stims:

My stims: These stims are played randomly, which means that in normal mode, when the Mistress wants to use a frustrating stim, she will randomly pick one from the 'Frustration' category. Same for the other categories (tease, edge, orgasm).
You can create (and then share publicly) as many stims as you want for the available categories, but also remove any stim you dislike.
Commons stims: These ones are specific stims that the Mistress will always play in certain scripts. For example: 'endOrgasm_final' is the very last stim played during the orgasm script, it's supposed to make you climax.
These stims can only be edited and are not sharable.
You can find a description of all these stims by clicking the "?" icon next to the 'Commons' category.

All stims can be altered during a session—they may speed up, slow down, or increase or decrease in intensity. Keep that in mind while crafting a stim.
Create a new stim:

On the screen above, I've created a new stim named 'my_frustration_stim' for the category: 'Frustration'.
Go to the following page.
Click the 'My created stims' tab. There you can find and manage all the stims you've created: publish them, add them to your collection, or delete them.

BUT, stims here won't be playable during a session yet.
You'll need to add them to your playable collection first. To do that, click the 'plus' icon to 'Add to your collection'.
Once you've done that, go to the page.

There you have a full view of all the playable stims within their respective categories.
Here, you can easily enable or disable any stims. By clicking the minus icon, that stim won't be played during a normal session, so you can remove any unpleasant stims for your settings.
You can see that the newly created stim is now displayed here, which means it can be played during a session.
If you think the 'endOrgasm_final' stim isn't very good and that you'll never be able to reach orgasm with it, you can just edit it to make a better version.

The "E" indicates that this common stim has been edited. You can easily revert to default by clicking the circular arrow next to it.