I would like to install Windows 10 in my second partition, but I don't want to break my ElementaryOS install.
I have all ready break my windows install (yesterday) so I don't want to make mistakes.
The Windows installation process generally does mess with Grub, the Linux bootloader. One way to prevent such issues is to install Windows in a virtual machine such as VirtualBox.
This answer has more information on how to install Windows and then get your Linux setup working again.
If you've already backed up your data, the simplest option might be to do a clean install of Windows, replacing elementary, then install elementary alongside Windows. This probably sounds like extra work, but in general it's easier to install Linux onto a Windows machine than it is to install Windows onto a Linux machine.
For more information, see Ubuntu's Windows Dual Boot guide.