How to change the front bumper and grill?

The LEMON repair guides online want much more specific information about the particular model of your Mustang; there are a lot more variations than you'd think. If you go to the 2018 Ford page and click in the Mustang folder, it comes up with all these options.

• EcoBoost Premium, 2D Convertible, Automatic Trans
• EcoBoost Premium, 2D Convertible, Standard Trans
• EcoBoost Premium, 2D Coupe, Automatic Trans
• EcoBoost Premium, 2D Coupe, Standard Trans
• EcoBoost, 2D Convertible, Automatic Trans
• EcoBoost, 2D Convertible, Standard Trans
• EcoBoost, 2D Coupe, Automatic Trans
• EcoBoost, 2D Coupe, Standard Trans

For the sake of this example, I picked the 2D Coupe with an automatic transmission; I suspect most of the information is going to be the same, but there could easily be differences, so you'll want to visit the one that matches your car the closest.

Anyway, here's the page showing how to remove the bumper cover, which is what I'm assuming you meant when you say you wanted to remove the front bumper; there is a separate section for the actual bumper itself if you really do want to remove it.

Exterior Body Panels: Bumpers — 2018 Ford Mustang EcoBoost, 2D Coupe, Automatic Trans Service Manual ~ LEMON Manuals

On that page click on the Disassembly and Assembly folder, then follow that with the proper Front Bumper Cover folder, depending on whether you have the SVT performance package or not.

It looks like most of the other information regarding the grille can be found on this page.

Front End Body Panels — 2018 Ford Mustang EcoBoost, 2D Coupe, Automatic Trans Service Manual ~ LEMON Manuals

Again, go to the Removal and Installation folder and under that the Active Grille Shutter folder has the removal and installation instructions.