Hi!
The openwrt wiki is pretty straight forward. Pick something that's
completely supported and easy to get a console on (ie, most of the
tplink hardware) and follow the instructions.
building openwrt is pretty simple on a linux box.
The most complicated bit is soldering the serial console and making
sure you have a 3.3v serial -> USB device that's actually doing 3.3v.
Then, yes, it's follow the instructions.
Tip: make sure the tplink you get (a) has an atheros CPU in it, (b)
isn't 11ac, as that stuff is still very new in openwrt/linux.
If you're after specific models: I've had a lot of success with new
people bringing up linux/freebsd on the tplink tl-wndr3600/4300, and
the tl-1043ndv2. They're all atheros parts, all the pieces inside are
supported, flashing openwrt is easy. The wiki pages for the units go
into a lot of detail.
HTH,
-adrian