Okay, so there's tons of information regarding this topic on the internet, but the general rules you should follow and information you should check before doing dual boot are the following:
The correct way of doing dual-boot Windows+Linux is to install Linux first. That's because once you install Linux, GRUB will manage the boot process and will provide you a screen so you can choose which Operating System you want to boot.
If for any reason windows is reinstalled, upgraded, etc. You`ll notice that GRUB won't appear anymore because Windows will take over as 'boot manager' and it does not understand that there's a Linux installed.
You need to understand and decide if you want to use legacy mode or UEFI (newer option, Windows 10 defaults to this). You can check which mode your Windows is running by n ways, e.g https://blogs.technet.microsoft.com/home_is_where_i_lay_my_head/2012/10/02/how-to-check-in-windows-if-you-are-using-uefi/. If your Windows is UEFI and you want to change to legacy, you need to reinstall everything. Dual boot works in both modes and there's advantages and disadvantages to both, check here: Are there any drawbacks to legacy BIOS as opposed to UEFI?
If you're using UEFI, the general rule of thumb when installing Linux (when Windows is also installed in UEFI) is, when installing GRUB, point to where`s the EFI partition is (this partition is used for UEFI to boot the system).
Generally, people have three issues when dual booting:
5.a GRUB does not appear at all, therefore you need to check if you`re booting correctly and if GRUB was installed (generally in this situations Windows boots normally). You can always reinstall GRUB.
5.b GRUB appears in rescue-mode, this can means a lot of things, one case I had I just needed to point to grub-rescue where the grub file was inside the EFI partition.
5.c GRUB is booting correctly but does not show an option to boot Windows. This is the easiest one (and probably was your case), because what happens is that all files from Windows are still there, you just need to point GRUB where they are. I already faced this issue and fixed with this link: GRUB does not detect Windows
When dual-booting, please, understand what the purpose is of each one of the partitions you have, this will help you doing a clean and working installation.
Backup your files if you are not confident in your ability.
If for some reason you can't boot or your system does not appear, don't panic, unless you made a really bad decision and deleted partitions or something like that, all the files are still there and you just need to point grub, the OS, or your BIOS to the right direction. Almost everything can be fixed without a complete reinstall.
Sometimes breaking the booting process is frustrating because you need to reboot thousand times, spend hours on Google, etc. But face that as a opportunity to learn.
If you don`t like to search, learn and break a few things in the way, stick to Windows.
Check this documentation about grub: https://help.ubuntu.com/community/WindowsDualBoot
Tecmint has a good tutorial: https://www.tecmint.com/install-ubuntu-16-04-alongside-with-windows-10-or-8-in-dual-boot/
I'll be happy to help if you face any issue in your new installation. Good luck!
