The default Manjaro installer looks as if it will allow you to define LVM partitions during the installation process but it actually won't. I like LVM a lot, and being determined to have a Manjaro system with LVMs underneath I spent some time researching and have what I think is a foolproof way of doing it.
I'm assuming here that you want separate root and home volumes; if you don't these instructions should still work, but you'll need to adapt them.
Install Manjaro on ordinary partitions
I'll leave you to work out the sizes, and to decide whether you need an EFI partition and so on.
Boot a live CD/USB medium
I like sysrescueCD.
Dump the root and home partitions to external storage
I use fsarchiver to do the dumping.
fsarchiver savefs /mnt/sdb1/prospero/prospero-manjaro-nolvm.fsa /dev/nvme0n1p5 /dev/nvme0n1p5
Delete the root and home partitions and set up a PV
I use parted. Something like this should work:
Model: UMIS RPITJ1TBVME2HWD (nvme)
Disk /dev/nvme0n1: 1024GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 1049kB 538MB 537MB fat32 boot, esp
2 538MB 1075MB 537MB ext4
3 1075MB 35.4GB 34.4GB linux-swap(v1) swap
4 35.4GB 193GB 157GB ext4
5 193GB 1024GB 831GB ext4
(parted) rm 5
(parted) rm 4
(parted) mkpart primary 35.4GB 100%
(parted) set 4 lvm on
Set up the volume group and the LVs
[root@sysrescue ~]# pvcreate /dev/nvme0n1p4
WARNING: ext4 signature detected on /dev/nvme0n1p4 at offset 1080. Wipe it? [y/n]: y
Wiping ext4 signature on /dev/nvme0n1p4.
Physical volume "/dev/nvme0n1p4" successfully created.
[root@sysrescue ~]# vgcreate vg01 /dev/nvme0n1p4
Volume group "vg01" successfully created
[root@sysrescue ~]# lvcreate -L 100G --name root vg01
Logical volume "root" created.
[root@sysrescue ~]# lvcreate -L 500G --name home vg01
Logical volume "home" created.
Restore the dumped partitions
fsarchiver restfs /mnt/sdb1/prospero/prospero-manjaro-nolvm.fsa id=0,dest=/dev/mapper/vg01-root id=1,dest=/dev/mapper/vg01-home
Mount the root partition
mount /dev/mapper/vg01-root /linux
Get the UUIDs of the partitions
blkid /dev/mapper/vg01-root /dev/mapper/vg01-home
Now use an editor (I like sed for this) to change the UUIDs for the root and home filesystems in /linux/etc/fstab to be the UUIDs displayed above. You may not need to make any changes.
chroot into the mounted root filesystem
(That "modprobe" is in there because of this Stack Exchange article.)
Mount required other filesystems and reinstall grub
mount /dev/nvme0n1p2 /boot
mount /dev/nvme0n1p1 /boot/efi
mount -t efivarfs efivarfs /sys/firmware/efi/efivars
Add LVM2 to the hooks in the initramfs
(Otherwise the LVM filesystems aren't seen by the kernel, which doesn't end well)
sed -i 's/base udev autodetect modconf block/base udev autodetect modconf block lvm2/' /etc/mkinitcpio.conf
The LVM2 hook must be after "block".