Step 1: Firstly, Right Click on drawable as shown in figure below.
Step 2: After that Go New > Drawable resource file as shown in figure below.
Step 3: After that New Resource file dialog box open on your computer screen. Enter File name in the text box and then press OK. Here File name is custom_button.
Important Note: File names must start with a lowercase letter.
Step 4: After that your drawable resource XML file will be created as shown in figure below.