Gah! Yeah, you wrote so a couple of posts above, sorry. @redp, I agree with @jeff, you should instead try the development snapshot.
I could not find TL-MR-3420 here https://downloads.openwrt.org/snapshots/targets/ath79/generic/
Do I have to clone the repo directly from git to get it?
Manage to upgade to 17, but can't seem to get SQM to work.
root@OpenWrt:~# uci set sqm.eth1.download='18000'
root@OpenWrt:~# uci commit sqm
root@OpenWrt:~# /etc/init.d/sqm restart
SQM: Starting SQM script: simple.qos on eth1, in: 18000 Kbps, out: 10000 Kbps
SQM: ERROR: QDISC htb is NOT useable.
Huge changes in SQM since 2017 -- it's even in the current Linux kernel now, from at least 4.19 forward (master, not openwrt-19.07)
Made an image.
How do I go about flashing it?
Do I use the factory or the sysupgrade image?
What are you running now? OpenWrt 17? If so, sysupgrade -n or LuCI and disabling keep settings should flash the "sysupgrade.bin" image.
Ok. Then I'll have to set everything up over ssh.
Any good guides for beginners?
And to install luci-sqm, how do I resolved the dependency issues I encountered before?
Since you're low on flash and are assembling your own images, just include them when you assemble your image. That said, with 4 MB of flash, you are unlikely to get LuCI to fit.
Is there anything "unique" about your connection to your ISP (PPPoE, VLANs, ...)?
If not, it will "just" be learning vi or how to use uci commands to edit the config files.
Basics of connecting with SSH are at https://openwrt.org/docs/guide-quick-start/sshadministration
Some useful links on the structure of those files are linked off of https://openwrt.org/docs/guide-user/base-system/start
People here should be more than willing to help with specific questions. I'd use search of the forum first, then, if you didn't find the answer, creating a new thread for the config question (as it is a different topic than making your own image).
I could not fit luci-app-sqm on the snapshot image.
I tried removing as many packages as possible.
I remade the 17 image and checked size. I could fit luci and luci-app-sqm by removing ipv6 packages and opkg.
v17 is known insecure and no longer supported. If you can use it to get a quick “sketch” of config (preferably without connecting to the Internet or enabling wireless), that might give you a leg up on using master/snapshot builds.
I'm trying to make a snapshot build that has luci-app-sqm package.
On snapshot I remove almost everything and still can not fit only luci-app-sqm
There seem to be some tricks, described here.
But they seem confusing for a begginer like me. When trying to run the make menuconfig commanrd all I get is an error.
make: *** No rule to make target 'menuconfig'. Stop.
Why luci if you don`t have space? 
You don't need lucy-app-sqm, only sqm-scripts.
Also, you don't have the menuconfig target in the image builder, only when compiling from the source.
Ok, so now I might be able to fit sqm on ath79.
I will make an image when I get home.
If all goes well my only trouble will be setting it uo over ssh.
I get the following error when trying to build an image penwrt-imagebuilder-ath79-tiny.Linux-x86_64
make image PROFILE=tplink_tl-mr3420-v2 CONFIG_IPV6=n PACKAGES=" -ip6tables -odhcp6c -kmod-ipv6 -kmod-ip6tables -odhcpd-ipv6only -opkg sqm-scripts"
Building images for ath79 - TP-Link TL-MR3420 v2
Packages: sqm-scripts base-files busybox dnsmasq dropbear firewall fstools iptables kernel kmod-ath9k kmod-gpio-button-hotplug kmod-ipt-offload kmod-usb-ledtrig-usbport kmod-usb2 libc libgcc logd mtd netifd ppp ppp-mod-pppoe swconfig uboot-envtools uci uclient-fetch urandom-seed urngd wpad-mini
Package list missing or not up-to-date, generating it.
Building package index...
Downloading http://downloads.openwrt.org/snapshots/targets/ath79/tiny/packages/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_core
Downloading http://downloads.openwrt.org/snapshots/packages/mips_24kc/base/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_base
Downloading http://downloads.openwrt.org/snapshots/packages/mips_24kc/luci/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_luci
Downloading http://downloads.openwrt.org/snapshots/packages/mips_24kc/packages/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_packages
Downloading http://downloads.openwrt.org/snapshots/packages/mips_24kc/routing/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_routing
Downloading http://downloads.openwrt.org/snapshots/packages/mips_24kc/telephony/Packages.gz
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/openwrt_telephony
Downloading file:packages/Packages
Updated list of available packages in /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/../../../../../../../home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/imagebuilder
Installing packages...
Installing libc (1.1.24-2) to root...
Installing libgcc1 (8.3.0-2) to root...
Copying /home/sila/Downloads/openwrt-imagebuilder-ath79-tiny.Linux-x86_64/dl/libgcc1_8.3.0-2_mips_24kc.ipk.
Collected errors:
* opkg_install_pkg: Package size mismatch: libgcc1 is 35737 bytes, expecting 35744 bytes
* opkg_install_cmd: Cannot install package libc.
make[2]: *** [Makefile:155: package_install] Error 255
make[1]: *** [Makefile:118: _call_image] Error 2
make: *** [Makefile:197: image] Error 2
Can you share your cooked firmware. I have v2 and all i need is SQM only. Thanks.