The Linksys WRT1900ACS is a popular router for running the OpenWRT firmware. It has been battle tested by the community and is a very robust platform.
All you need to flash OpenWRT on the router is a PC and an Ethernet cable. If your PC doesn't have an Ethernet socket then get a USB to Ethernet adapter.
Never flash the router over a wireless connection.
Go to OpenWRT's Table of Hardware.
Search for wrt1900 in the Model column.
Follow to link to the Device Page of linksys_wrt1900acs.
Download the file linked in the column "Firmware OpenWrt Install".
Flashing the router could brick it! Double-check you downloaded the correct firmware for your exact hardware version.
Connect your PC to a LAN port on the router with an Ethernet cable.
The radios are disabled after flashing the router. The only way to configure them is over a wired connection.
You can leave the Internet cable connected to the Modem or Media Converter provided by your ISP, but it is not required at this point.
Open the OEM firmware in a browser at http://192.168.1.1/.
Sign in with the router password you saved during the guided setup of the OEM firmware. If you set up the OEM firmware manually then try 'admin'.
Navigate to the Connectivity page.
Click Choose File and select the OpenWRT firmware downloaded previously. Then click Start.
Click to continue when you get a warning of "Unrecognized file name". Be patient while it's flashing.
In the reboot popup click Yes.
Wait for the Power led to stop blinking.
Open the OpenWRT firmware at http://192.168.1.1/ in a new browser window.
Navigate to System -> Administration and change the root password.
Navigate to Network -> Wireless.
The Linksys WRT1900ACS is a dualband router. It has a radio for the 2.4 GHz and the 5 GHz band.
Radio0 operates the 5 GHz band for the 802.11n or ac wireless protocols.
Radio1 operates the 2.4 GHz band for the 802.11b, g or n wireless protocols.
Each radio in the Wireless Overview table is followed by a row with the wireless network the radio is associated with.
Click the Edit button in the row below the radio0 Marvell 88W8864 802.11nac.
In the General Setup tab set Mode to "AC", Channel to "Auto" and Width to "80 MHz".
In the Advanced Settings tab select you own country in the Country Code field to comply with national regulation for use of radio frequencies.
In the bottom General Setup tab enter a new name for you 5 GHz wireless network in the ESSID field.
In the bottom Wireless Security tab set Encryption to "WPA2-PSK (string security)".
Enter a long key for you 5 GHz wireless network, preferably a passphrase of 3 to 4 random words.
Check "Enable key reinstallation (KRACK) countermeasures".
Back in the Wireless Overview page click Save and Apply at the bottom of the page and then click Enable on the 5 GHz wireless network you just configured.
It takes about half a minute before the radio becomes active (you don't have to click restart).
In the Wireless Overview click the Edit button in the row below the radio1 Marvell 88W8864 802.11bgn.
In the General Setup tab set Mode to "N", Channel to "Auto" and Width to "20 MHz". 802.11n is backwards compatible with 802.11b and g.
Enter a new name for the wireless network in the ESSID field and apply the the same settings for Country Code and wireless security as the 5 GHz band.
Back in the Wireless Overview click Save and Apply, and then click Enable on the 2.4 GHz wireless network you just finished configuring.
Wait about half a minute for radio1 to activate.
Then you are done.