I use https://github.com/debian-pi/raspbian-ua-netinst
But you may find it easier just to use the Raspbian lite image. Once you've got the base sorted be sure to..
# Update the system..
sudo apt update;sudo apt upgrade
# Use latest kernel..
apt install rpi-update; sudo rpi-update
# Reboot to start using new kernel..
reboot
After you've flashed the image & your box is up & running. Plug in your device(s) and check..
dmesg
To make sure it initialises the tuner, you should see something like this in dmesg.
si2168 21-0064: Silicon Labs Si2168 successfully attached
si2157 22-0060: Silicon Labs Si2147/2148/2157/2158 successfully attached
Make sure you have the latest firmware (it won't actually be used until you attempt to tune).
# Switch to root.
sudo -s
# cd to firmware dir.
cd /lib/firmware
# Download firmware to /lib/firmware.
wget http://palosaari.fi/linux/v4l-dvb/firmware/Si2168/Si2168-B40/4.0.25/dvb-demod-si2168-b40-01.fw
# Escape root.
exit
To install & build Tvheadend follow my guide here: https://tvheadend.org/d/1641-how-to-compile-tvheadend-for-raspberry-pi?r=21384#message-21384
As in the post, make sure you don't build ffmpeg or you'll be waiting forever, it's pointless on the Pi anyway. If you get an error about dvb scan tables failing to download pass --disable-dvbscan to the build script, so..
AUTOBUILD_CONFIGURE_EXTRA=--enable-hdhomerun_client\ --disable-ffmpeg_static\ --disable-dvbscan ./Autobuild.sh
The choice of stable or unstable is yours. Go for stable if you want well, extremely stable, or unstable (which is actually pretty stable but can bug out under certain conditions) if you want newer features.
Just keep an eye out on https://github.com/tvheadend/tvheadend for commits to the branch and ..
cd /path/to/source
git pull
.. Any changes.
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If you want to mount a usb drive for recordings, see http://www.raspberrypi-spy.co.uk/2014/05/how-to-mount-a-usb-flash-disk-on-the-raspberry-pi/.
Just don't chown the main directory as the pi user, instead (as root) make recording/timeshift buffer dirs & chown it as the hts user, example..
sudo -s
cd /path/to/drive
mkdir /path/to/drive/tvheadend/dvr /path/to/drive/tvheadend/buffer -p
chown hts:hts /path/to/drive/tvheadend/ -R
exit
You must replace /path/to/drive with the actual path that you've chosen.
You can then just point tvheadend to those dirs in your recording profile & timeshift config.
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Hopefully this crash course should get you started, I may have missed something - I've wrote this on 2 hours sleep, If I have, sorry!