0. Login as root or use sudo.

1. Install layman to manage overlays if don't have this tool already.

sudo emerge -v app-portage/layman

2. Add brancik-overlay.xml file to the /etc/layman/overlays directory with the following content:

sudo nano -w /etc/layman/overlays/brancik-overlay.xml

        brancik-overlay
        Ctibor Brancik's overlay
        http://git.brancik.cz
        
                ctibor@brancik.cz
                Ctibor Brančík
        
        git://git.brancik.cz/gentoo-overlay.git

Or you can use layman-overlay-maker tool if you have >=app-portage/layman-2.0.0.

More info about Gentoo overlays and their management can be found on Gentoo wiki.

3. Add overlay:

sudo layman -f -a brancik-overlay

4. To install all available indilib drivers simply use provided portage set:

sudo emerge -v @indilib-drivers

5. If you want to install only drivers for the devices you use (as is the Gentoo way), you have to set appropriate USE flags:

Available USE flags (note that only 3rd party drivers are present, with exception of indi-qhy-old. All other drivers are installed by sci-libs/indilib itself):

    aagcloudwatcher
    apogee
    asicam
    duino
    eqmod
    fishcamp
    fli
    gphoto2
    maxdomeii
    meadedsi
    qhy
    qsi
    sbig
    spectracyber
    sx
    tess

nano -w /etc/portage/package.use/indilib

#Enable drivers USE flag which pulls sci-libs/indilib-drivers as dependency

sci-libs/indilib drivers

# To install indi_gphoto_ccd, indi_qhy_ccd and indi_sbig_ccd.
# sci-libs/indilib-drivers is a meta ebuild and works similar to x11-base/xorg-drivers package.

sci-libs/indilib-drivers gphoto2 qhy sbig

sudo emerge sci-libs/indilib::brancik-overlay

6. Note that ebuilds for stable release of indilib drivers (currently 1.0.0) are keyworded unstable (~amd64, ~x86 specifically) and live source ebuilds are all masked by package.mask. You will have to unmask them during installation (use emerge with --autounmask-write option). See official docs if you do not know this means exactly.