How to create custom font family with bold font

Set 700 as the fontWeight value for your font in bold.

As it's in the font resources documentation:

The most common values are 400 for regular weight and 700 for bold weight.

In the lobster font family example you linked it would be:

<!-- bold -->
<font
    android:fontStyle="normal"
    android:fontWeight="700"
    android:font="@font/lobster_bold" />

And, if you are using support lib:

<!-- bold -->
<font
    app:font="@font/lobster_bold"
    app:fontStyle="normal"
    app:fontWeight="700" />