Can't find the instant alpha tool in "Preview?" OS 10.13.2
Open Preview
Open Image you wish to edit
Click the "Mark Up" icon located beside the search bar, this will open the former "toolbox" in the older version of Preview.
You will just have to hover over each icon you'd like to find the correct one.
The Instant Alpha Tool looks like a magic wand, located beside the shapes icon or square far left.
This should solve your issues.
Hold the "Command" key and select the area you want to delete, then hit the delete button and when asked hit the "convert" button
null-OTHER, macOS High Sierra (10.13.3), Preview - Instant Alpha