After booting a disk and displaying a message, the second stage is to load the second sector from the disk.  Then jump to the start address where the sector was loaded in memory.

[org 0x7C00]
[bits 16]

start:
  cli
  xor ax, ax
  mov ds, ax
  mov es, ax
  mov ss, ax
  mov sp, 0x7c00
  sti

  mov ah, 2
  mov al, 1
  mov bx, second
  mov cl, 2
  mov ch, 0
  mov dh, 0
  int 0x13
  jmp second

times 510 - ($ - $$) db 0
dw 0xaa55

second:
  mov ah, 0x0e
  mov al, 'X'
  int 0x10
  hlt

times 1024 - ($ - $$) db 0

Assemble the file:

$ nasm -f second.s -o second.bin

Then write the two sectors to the disk.