It will help greatly to go into this guide with some understanding of C and Assembly.

Before you start you will need to install nasm, genisoimage and may want to install qemu or bochs in order to test your OS without rebooting your computer, bochs is preferred as it has more debugging capability than qemu

By the end of this part you should have a very basic OS that can boot.
This part of the guide will not cover anything but making a basic OS that does absolutely nothing due to how long it already is.
If people like this I will make a part 2 going over how to use C with the OS and write text to the screen.

Booting

The first thing to do is create the main assembly file, I will call it Boot.asm

global entry

MAGIC equ 0x1BADB002 ; These tell the bootloader that we are bootable
FLAGS equ 0
SUM equ -MAGIC

section .text:
align 4 ; Align everything after this to a 4 byte boundary, which the boot header needs to be aligned to
        dd MAGIC ; dd means "define double word", a word is usually 2 bytes on most computers, so a dword is 4 bytes. You can think of a word as being a short in C and a dword being an int.
        dd FLAGS
        dd SUM

entry:
jmp entry ; For now we won't do anything but loop forever.

This should boot and do nothing.

But how do we compile it?

First we need a linker script since grub(the bootloader this tutorial will be using) is loaded below 1 MB so we should load above that!

Now I don't expect you to know how to do this, and neither do I, so I will copy this from littleosbook which is another good tutorial on writing an operating system from scratch.

ENTRY(entry)                /* the name of the entry label */

SECTIONS {
    . = 0x00100000;          /* the code should be loaded at 1 MB */

    .text ALIGN (0x1000) :   /* align at 4 KB */
    {
        *(.text)             /* all text sections from all files */
    }

    .rodata ALIGN (0x1000) : /* align at 4 KB */
    {
        *(.rodata*)          /* all read-only data sections from all files */
    }

    .data ALIGN (0x1000) :   /* align at 4 KB */
    {
        *(.data)             /* all data sections from all files */
    }

    .bss ALIGN (0x1000) :    /* align at 4 KB */
    {
        *(COMMON)            /* all COMMON sections from all files */
        *(.bss)              /* all bss sections from all files */
    }
}

nasm -f elf32 Boot.asm should create a file called Boot.o

Which we can then turn into an elf binary by linking it with ld

ld -T link.ld -melf_i386 Boot.o -o kernel

We then must download grub, which can also be found in littleosbook, http://littleosbook.github.com/files/stage2_eltorito

Now we create the folder structure our os iso will be built from

mkdir iso
mkdir iso/boot
mkdir iso/boot/grub
cp FILE_FROM_LITTLEOSBOOK iso/boot/grub

And our grub configuration to put in /boot/grub/menu.lst

default=0
timeout=0

    title Replit
    kernel /boot/kernel

Now to run it!

A bash script to automatically compile/link and run the OS

nasm -f elf32 Boot.asm
ld -T link.ld -melf_i386 Boot.o -o iso/boot/kernel

genisoimage -b boot/grub/stage2_eltorito -R -boot-load-size 4 -no-emul-boot -boot-info-table -A Replit  iso > os.iso

# Uncomment for Qemu
# qemu-system-x86_64 -kernel iso/boot/kernel
# Uncomment for Bochs
# bochs -q

To use bochs you must make a configuration file named bochsrc

megs:            32
romimage:        file=/usr/share/bochs/BIOS-bochs-latest
vgaromimage:     file=/usr/share/bochs/VGABIOS-lgpl-latest
ata0-master:     type=cdrom, path=os.iso, status=inserted
boot:            cdrom
log:             bochslog.txt
clock:           sync=realtime, time0=local
cpu:             count=1, ips=1000000