Mini guide to build an ISO
Prerequisites
- lots of diskspace (skip this if your root is sufficiently large)
- installing the tools
- tools configuration
- building
Diskspace
You need a lot diskspace - 15-20G per iso you intend to build
The tools
Install the manjaro-tools-packages requred.
sudo pacman -Syu --needed git base-devel manjaro-chrootbuild manjaro-tools-iso-git manjaro-tools-yaml-git manjaro-tools-base-git
Configure tools
With the tools installed copy the default tools configuration to your home
cp -R /etc/manjaro-tools ~/.config
Modify the tools configuration to point to your storage partition’s mount point - use your favorite editor to edit the file
micro ~/.config/manjaro-tools/manjaro-tools.conf
Then use git to clone the iso profiles repo in your home
git clone https://gitlab.manjaro.org/profiles-and-settings/iso-profiles
Create a config file ~/.config/manjaro-tools/iso-profiles.conf with content
run_dir=/home/$USER/iso-profiles
Building
The building can be as simple as
buildiso -p lxqt
If you did not change the paths in manjaro-tools.conf the resulting lxqt ISO is stored at
/var/cache/manjaro-tools/iso/community/lxqt
Building OEM iso
The profiles for the various hardware vendors supporting Manjaro is located in the oem folder of the iso-profiles.
There you will find profile for minisforum and other OEM branded profiles - e.g. UM350
buildiso -p minisforum-oem-kde-um350
Build an ISO from current system
There is no GUI to create an ISO from current state.
Technically you can use buildiso so I have outlined the commands necessary
I recommend to test them as they are written from memory - flash memory - the kind of memory which is gone in a flash 
- after you have tested them you create a script to do lifting for you.
Steps
1. Create a folder in the root filesystem to hold your system
sudo mkdir /mybuild
2. Set permissions on the folder
Setting your user as owner which automagically gives rw access
sudo chown $USER:$USER /mybuild
3. Clone the iso-profiles repo into /mybuild
git clone https://gitlab.manjaro.org/profiles-and-settings/iso-profiles /mybuild/iso-profiles
4. Create a folder to hold your build
mkdir /mybuild/iso-profiles/$USER
5. Copy the relevant profile-folder into mybuild
In this case it is Plasma - it would work for any edition - ensure a unique name the folder - it must be unambigious across iso-profiles subfolders
cp -R /mybuild/iso-profiles/manjaro/kde /mybuild/iso-profiles/$USER/my-kde
6. Clean up your system for orphans
pamac remove --orphans
7. Create a package list of your current system
Excluding custom packages
pacman -Qqen > ~/my-packages.txt
8. Create new filtered list
Use the comm utility comparing the two files ~/my-packages.txt and /rootfs-pkgs.txt and pipe the result to replace the my-kde profile’s Packages-Desktop file
comm -23 <(sort ~/my-packages.txt) <(cat /rootfs-pkgs.txt | cut -d' ' -f1 | sort) > /mybuild/iso-profiles/$USER/my-kde/Packages-Desktop
9. Create the folder to hold your personal settings
mkdir -p /mybuild/iso-profiles/$USER/my-kde/desktop-overlay/etc/skel
10. Copy the content of your home
Close all open applications - apps which may keep files open in relevant folders. E.g. browsers and mail clients.
Copy the content of your home into the desktop-overlay/etc/skel folder - make sure to be selective and remove unwanted files like Documents and Pictures and whatnot - watch out for hidden files and folders e.g. ~/.mozilla
cp -R $HOME/. /mybuild/iso-profiles/$USER/my-kde/desktop-overlay/etc/skel
11. Run buildiso
Use the -t target directory argument to place your build inside the /mybuild folder.
If you have used buildiso previously with another profile-root you need to edit the file ~/.config/manjaro-tools/iso-profiles.conf to point to the folder /mybuild/iso-profiles
buildiso -p my-kde -t /mybuild
The size of the final ISO will be reflected by the packagelist and the size of your $HOME folder
More reading
More info can be found at the wiki - including how to include your own web based package repository