i've had a quick look at itsfoss and it raised a couple of queries in my head.
When you install a linux distribution there are the factors of root install and home regarding the partitions. But also there are the factors of the Linux kernel and setting it up so that on boot your PC knows what OS's are available where the Linux kernel is eg in my Boot directory i have.
EFI/ grub/ initramfs-linux-fallback.img* initramfs-linux.img* vmlinuz-linux*
Now then here is my problem unless i didn't fully read its foss. When you installed say Mint , the install put Mint 20.* and everything else where it should; when you boot the bootloader i.e GRUB will look for that Mint kerenel. The way itsfoss article reads is they talk about the partitition but they have not covered the point about re-installing grub. if your Ok with grub thats fine but it must be re-installed otherwise on boot it will look for a Mint kernel thats no longer there .
The approach of selecting a manual install i agree; however the way i would go is before I start that process i would do the setting up of partitions using gParted from a live OS ; then it would just be the case of confirming partitions. In other words ,you use the gparted tool from a live OS, then you can click on the install icon.
I've had a look at booting manjaro live from:
https://manjaro.org/downloads/official/xfce/
manjaro-xfce-21.0.7-210614-linux510.iso was 2.6 gig and booted no problem - i went the Ventoy.net route. In the menu gParted is available as is screen shot. You can play with manjaro live; edit your pc partitions if you want and you see that icon in image - bottom left , you just click that and it starts the install
So now it come down to the steps ; you can use itsfoss ; which of the steps have you done so far & which do you think might be a problem?