How to install Ubuntu and Windows on one machine with dual boot.
Just install both
The easiest way is to first install Windows, and then Ubuntu. Ubuntu comes along with a boot screen where you can select which OS (Operating System) you want to boot . By default, if you don't make a choice in the bootscreen, after X seconds, Ubuntu will be booted by default, but you can change that in the boot manager or disable the timer entirely.
But how to install the OS ?
First, you need to download an Ubuntu and Windows iso. Next, you need also a Windows License. I would recommend to buy an ORM Licence for 3-4 €.
Next, the question arises, how to install it. If you happen to have an DVD burner, you could use a DVD to copies the OS images. However, next time you want to reinstall Windows/Ubuntu, you will need a new DVD, so you basically will just use that DVD once and then its already outdated. Also, most new laptops do not even shit with a DVD drive anymore. Therefore, I would recommend to install the OS on an USB stick.
Ignore Startup Disk Creator
The official guide lines of Ubuntu recommend to install the iso image on an USB stick using the Startup Disk Creator. This has a big downside, because you can't use the USB stick for anything else. Also the process takes time, and next time in 2 years when you want to install Ubuntu, you need to do the process again.
Also for Windows, creating a bootable USB has become more complicated now. As most systems use UEFI (allows you to use the mouse in bios and boots faster), one has to partition the USB stick with FAT32. However FAT32 comes with a max-size for files to 4GB. However, since April 2021, the file "install.wim" from that Windows 10 iso is 5.2 GB, so it cannot be copied to a plain FAT32 USB stick any longer. But whenever a door closes, a new one opens. And there is a new player who helps us witht his problem, and makes the install process just much more fun!
Ventoy will partition your USB stick in two partitions. One is FAT32 and will handle the boot process. The other partition, will just contain the iso images (compressed). So all you have to do, is to download Ventoy (can be used at Windows and Ubuntu), and partition your USB drive. Then, just copy the Windows and Ubuntu iso to the second partition. This means, there are only two files on that partition, you do not have to extract the isos!
Then, just boot your computer from USB. A boot loader will appear and ask you, which OS (depending on what you have on your USB) you want to install.
This makes the installing process a pure joy. In future, a new release has just to be downloaded and copied to the USB stick and is ready for install!
You can find a good guide how to install Venoty actually at https://superuser.com/a/1758255/236337.
Having Ubuntu and Windows on two different disks
I prefer to have Ubuntu and Windows installed on two different disks. This makes it easier, when one wants to reinstall one of the two. However, the process of installing Ubuntu is slightly more complicated, because when you get asked where you want to install Ubuntu, you only get the option to erase your Windows partition or install Ubuntu along side. Installing Ubuntu on a different partition is not a default option. Thus you have to pick "Somethign else" and setup EFI System partition, swap partition etc. all by yourself. A very good guide how to do this can be found at https://askubuntu.com/a/1130381/247771 under the section "Install Ubuntu".