BIOS Settings
Enable Intel VT-d or AMD-Vi in BIOS settings. If these options are not present, it is likely that your hardware does not support IOMMU.

Disable Resizable BAR Support in BIOS settings. Cards that support Resizable BAR can cause problems with black screens following driver load if Resizable BAR is enabled in UEFI/BIOS. There doesn't seem to be a large performance penalty for disabling it, so turn it off for now until ReBAR support is available for KVM.

Set the kernel paramater depending on your CPU.

grub-mkconfig -o /boot/grub/grub.cfg

Reboot your system for the changes to take effect.

To verify IOMMU, run the following command.

dmesg | grep IOMMU

The output should include the message Intel-IOMMU: enabled for Intel CPUs or AMD-Vi: AMD IOMMUv2 loaded and initialized for AMD CPUs.

To view the IOMMU groups and attached devices, run the following script:

#!/bin/bash
shopt -s nullglob
for g in `find /sys/kernel/iommu_groups/* -maxdepth 0 -type d | sort -V`; do
    echo "IOMMU Group ${g##*/}:"
    for d in $g/devices/*; do
        echo -e "\t$(lspci -nns ${d##*/})"
    done;
done;

When using passthrough, it is necessary to pass every device in the group that includes your GPU.
You can avoid having to pass everything by using ACS override patch.

systemctl enable --now libvirtd

rc-update add libvirtd default
rc-service libvirtd start

Sometimes, you might need to start default network manually.

virsh net-start default
virsh net-autostart default

NOTE: You should replace win10 with your VM's name where applicable
You should add your user to libvirt group to be able to run VM without root. And, input and kvm group for passing input devices.

usermod -aG kvm,input,libvirt username

Download virtio driver.
Launch virt-manager and create a new virtual machine. Select Customize before install on Final Step.
In Overview section, set Chipset to Q35, and Firmware to UEFI
In CPUs section, set CPU model to host-passthrough, and CPU Topology to whatever fits your system.
For SATA disk of VM, set Disk Bus to virtio.
In NIC section, set Device Model to virtio
Add Hardware > CDROM: virtio-win.iso
Now, Begin Installation. Windows can't detect the virtio disk, so you need to Load Driver and select virtio-iso/amd64/win10 when prompted.
After successful installation of Windows, install virtio drivers from virtio CDROM. You can then remove virtio iso.

Remove Channel Spice, Display Spice, Video QXL, Sound ich* and other unnecessary devices.
Now, click on Add Hardware, select PCI Devices and add the PCI Host devices for your GPU's VGA and HDMI Audio.

Libvirt hooks automate the process of running specific tasks during VM state change.
More info at: PassthroughPost

Note: Comment Unbind/rebind EFI framebuffer line from start and stop script if you're using AMD 6000 series cards (#9). Also, move the line to unload AMD kernal module below detaching devices from host. These might also apply to older AMD cards.

mkdir /etc/libvirt/hooks
touch /etc/libvirt/hooks/qemu
chmod +x /etc/libvirt/hooks/qemu