Topics:
- Lock the Grub2 boot menu
- Change the GRUB2 Splash
- Edit Grub2 menu entries
- Hide the grub menu
- Repair Grub
- Dual Boot


- Lock the Grub2 boot menu:

This a very simple to way to lock the Grub2 boot menu, visible immediately after unlock the crypt password when the system starts.

Preflight:
First of all back-up the files involved on every step.
Choose a Grub2 root user (MYUSER, not necessarily the system root), and a password but if you have chosen the root don't use the root password for the Grub2 configuration.

Steps:
Insert the next snippet at the end of the file /etc/grub.d/00_header:
cat << EOF
set superusers="root"
password root PASSWORD_HERE
EOF

To setup one user:
To setup the user root:
$ sudo sed -i -e '/^menuentry /s/ {/ --users root {/' /etc/grub.d/10_linux /etc/grub.d/10_linux /etc/grub.d/10_linux /etc/grub.d/30_os-prober /etc/grub.d/40_custom

The resultant lines in the code of the files should have added: --users root

UNDO the changes:
$ sudo sed -i -e '/^menuentry /s/ --users MYUSER[/B] {/ {/' /etc/grub.d/10_linux /etc/grub.d/20_memtest86+ /etc/grub.d/30_os-prober /etc/grub.d/40_custom

Note that there are types of commas ("", '', ``, ‘’) and dashes (-, -- ==> This is two single dashes).

Save the new changes with:
# update-grub
Don't run the command anymore, otherwise the changes made will be restored to default.

Reboot the machine to make sure the Grub2 editor has been locked.

References:
Web: http://www.dedoimedo.com/computers/grub-2.html
File: GRUB 2 bootloader - Full tutorial.pdf

PENDING:
I need to improve this process to add a second user and avoid password prompts in the Grub2 screen.

If you reboot at this point the the Grub2 splash is going to ask for the password every time you power on the machine. To skip the recurrent password prompt just replace Grub2 user roo by adding the user esteban:

Add esteban to /etc/grub.d/00_header:

cat << EOF
set superusers="root"
password root PASSWORD_HERE
password esteban PASSWORD_HERE
EOF

$ sudo sed -i -e '/^menuentry /s/ {/ --users esteban {/' /etc/grub.d/10_linux /etc/grub.d/10_linux /etc/grub.d/30_os-prober /etc/grub.d/40_custom

If you open the files and search '--users' should look lines like this:
$_get_device_id "${DEVICE}")'  --users root --users esteban {

- Change the GRUB2 Splash:

Change the image /boot/grub2/.background_cache.png which is mentioned in the file grub.cfg in the same directory.
To change the text lines:
$ sudo nano /boot/grub/grub.cfg
Press CTRL + X to search the string "background_image"
If you changed the image name or location remember to update GRUB.
References:
Web: https://wiki.debian.org/Grub/SplashImage
File: Grub_SplashImage - Debian Wiki.pdf

- Edit Grub2 menu entries:

Change the default boot option... 
(Pending)

- Hide the grub menu:

Backup and then edit this file:
$ sudo nano /etc/default/grub
Change the line:
GRUB_TIMEOUT=0
Save changes.
Update the program:
$ sudo update-grub
References:
Web: http://unix.stackexchange.com/questions/137856/how-hide-the-grub-2-start-up-screen-in-debian-during-bootup
File: grub2 - How hide the grub 2 start up screen in debian during bootup_ - Unix & Linux Stack Exchange.pdf

- Repair Grub:

(Pending)
References:
Web: https://www.debian.org/releases/stable/i386/ch08s07.html.en
File: 8.7 Recovering a broken system.pdf
Web: https://feeding.cloud.geek.nz/posts/reinstalling-grub-on-unbootable-debian/
File: Reinstalling grub on an unbootable Debian system.pdf
Topic: System rescue
Guide: System Rescue
Topic: Reset root password with Grub2
Guide: Reset root password

- Dual Boot:

Just say that if you have a dual boot with Windows the time will be affected and you will have to correct the issue.
References:
Guide: Set date, time and timezone

Every non Linux system has his installation issues out of the objectives of this book, so the only thing I say is use a good partition software like Gparted
References: 
Guide: Partitioning