Linux Real time kernel build- Raspberry PI 4B module

Hi I was able to build and install the Real time kernel,
though i am not sure my steps were correct.

Current Script:

sudo apt install git bc bison flex libssl-dev make libncurses5-dev #I think this is all the tools required
mkdir kernel
cd kernel/
git clone --depth=1 --branch rpi-6.1.y https://github.com/raspberrypi/linux
wget https://mirrors.edge.kernel.org/pub/...-rt27.patch.gz
cd linux/
zcat ../patch-6.1-82-rt27.patch.gz | patch -p1 --dry-run#check the patch fits
zcat ../patch-6.1-82-rt27.patch.gz | patch -p1
KERNEL=kernel8
make bcm2711_defconfig
make menuconfig#General -> Preemption Model select Real Time option
vi .config#add personalised suffix to CONFIG_LOCALVERSION
make -j4 Image.gz modules dtbs
echo $KERNEL
sudo make modules_install
sudo cp arch/arm64/boot/dts/broadcom/*.dtb /boot/
sudo cp arch/arm64/boot/dts/overlays/*.dtb* /boot/firmware/overlays/
sudo cp arch/arm64/boot/dts/overlays/README /boot/firmware/overlays/
sudo cp arch/arm64/boot/Image.gz /boot/$KERNEL.img

how ever, i have to included a line "kernel=\kernel8" in the /boot/firmware/config.txt .

when I reboot, i can get it as PREEMPT RT in ( uname -r)

Now I gave one issues
when I used "sudo apt install linux-headers-$(uname -r),

i am getting following error "unable to locate the package linux-headers-"

how ever I tried the following steps:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

Still having same issues.

how do we ensure the headers files are included before build or during the the installation.

Thanks
Ramesh.