First of all, check openresolv package is installed in your system or not. if not then simply install it by:

sudo pacman -S extra/openresolv

Step 2: Remove Immutable Permissions

# Check for immutable permissions on /etc/resolv.conf
lsattr /etc/resolv*

# If immutable permissions exist, them remove it by
sudo chattr -i /etc/resolv.conf

Step 3: Set Custom DNS Servers

# Edit the resolvconf.conf file
sudo nano /etc/resolvconf.conf

# Configuration for resolvconf(8)
# See resolvconf.conf(5) for details

resolv_conf=/etc/resolv.conf
# If you run a local name server, you should uncomment the below line and
# configure your subscribers configuration files below.
#name_servers=127.0.0.1
###### Block malware with 1.1.1.1 for Families #####
name_servers=1.0.0.2
name_servers=1.1.1.2

###### Use 1.1.1.1 resolver ######
name_servers=1.0.0.1
name_servers=1.1.1.1

###### AdGuard DNS ######
# name_servers=94.140.14.59
# name_servers=94.140.14.49

######### AdGuard Non-filtering DNS #######
name_servers=94.140.14.141
name_servers=94.140.14.140

Enable dhcpcd services to work Custom DNS. if it is not installed in your system then install it by

sudo pacman -S extra/dhcpcd 
sudo systemctl enable --now dhcpcd.service

Conclusion: You’ve successfully configured custom DNS servers on Arch based Linux. Enjoy improved network security and privacy with your chosen DNS servers.