Use custom XSL for the default view of a list (Allitems.aspx)

I personally wouldn't touch SharePoint Designer for this when it can mostly be achieved through the SharePoint GUI.

If you navigate to your list view page, you will be able to edit the page and it will behave exactly like a normal web part page. You'll see the List View Web Part on the page which you can bring up the web part properties for. Then in the Miscellaneous group, you'll see XSL Link where you can apply your already existing XSL file.

At least doing it this way, if you need to reverse the change you can edit the web part again and remove the XSL reference, whereas doing it through SharePoint Designer can often lead to "funnies".