Its normaly this simple: https://openwrt.org/toh/tp-link/td-w8970_v1#installation_using_serial
You just solder 3 or 4 pins (i always solder 4 pins - looks better for me). I have done this on a two digit number of devices. I never had any issues with falling off soldering pins ( @summers ). https://openwrt.org/toh/tp-link/td-w8970_v1#serial
Just copy the latest sysupgrade file from here https://downloads.openwrt.org/releases/18.06.1/targets/lantiq/xrx200/openwrt-18.06.1-lantiq-xrx200-tplink_tdw8970-squashfs-sysupgrade.bin to your tftp folder. But rename it! Name it something short. For example 8970.bin.
Then the command line looks like this: tftpboot 0x81000000 8970.bin
I recommend using minicom with flow control disabled and anything else default. https://openwrt.org/docs/guide-user/installation/generic.flashing.tftp?s[]=tftpd
Modern tftpd-hpa have its tftp directory set at /srv/tftp (not the named directory in the wiki). You can see the directory used when you run /etc/init.d/tftpd-hpa status .
For 8970 devices (without the terrible 5gh wifi chip) you take the image i linked above.
For 8980 and 9980 (with the terrible 5ghz non working chip) you take this image (or at later time any updated link) https://downloads.openwrt.org/releases/18.06.1/targets/lantiq/xrx200/openwrt-18.06.1-lantiq-xrx200-tplink_tdw8980-squashfs-sysupgrade.bin