A quick one:

You may have seen this message if you have tried to start Automatic1111:
"No module 'xformers'. Proceeding without it."

Since some modules seem to need xformers (dreambooth for example, I think),
What is the correct way to install or use xformers in Automatic1111 ?

This method works for me with version 1.7.0 of Automatic1111 and for a Windows system:

In "File Explorer", go to the "stable-diffusion-webui" folder.

Double check that you have a webui.bat file in the folder.
(If not there is already a problem).
In the location bar, type "cmd" and type enter.

windows explorer folder

This is supposed to open a command line window in the "stable-diffusion-webui" folder.

Now type:

webui.bat --xformers

If needed, this will install the files that you are missing to use xformers, then it will start
automatic1111 with the argument --xformers

Do that, each time you need xformers or create a new text file (with notepad), and add inside:

webui.bat --xformers

And give a .bat extension to this text file (and the name that you want).

To run Automatic1111 with xformers, you need then to just double click on this bat file.