Are there some instructions or Screenshots available where you can take a look at before you are flashing the router with this firmware?
Looks like you need telnet to configure the router from then on. Is it not possible via webbrowser anymore?
OpenWrt is mainly command-line driven. There is a web interface, though it's not installed by default so at a bare minimum, you would need to telnet/ssh and install that (opkg update; opkg install install luci-admin-full) before you could use it. The web interface covers much of the standard router features, but certainly does not cover the full range of what OpenWrt is capable of. For example, configuring your PPPoE information or NTP is fine, but I didn't see anything in there for my IPv6 configuration and I doubt it would have stuff for Asterisk. I found myself using a shell for installing new components (opkg), setting my wireless mode (enabling 'n' mode, but not setting the encryption, band, etc), and setting my IPv6 info. PPPoE, static DHCP, NTPD, and WiFi settings (all but setting the mode) I did from the web interface. One other thing of note, is the LEDs are not configured correctly (at least not with the firmware from a couple days ago). They can be configured from the web interface, but since my router's in a whole other room I haven't bothered.
In short, if you're not comfortable with a CLI, then I wouldn't recommend going the OpenWrt route (at least not yet). The good news is that the hard work is done, so hopefully it won't be long for more consumer oriented firmwares to add support as well (such as Tomato and DD-Wrt). On the other hand, if you're comfortable with a CLI and have a bit of time, then it's a good choice.
Although not a step-by-step picture instructions you asked about, some screenshots can be found here: http://wiki.openwrt.org/doc/configuring to give an indication of what the web UI looks like.
HTH.