Steps to install ubuntu alongside windows 10. Dual booting seems to be becoming more and more difficult. It goes without saying, you should backup everything before trying this because things can go wrong (very wrong)!
If you have bitlocker you need to save or print out your bitlocker key, this is very important, you will probably need to enter this at least once
Now shrink the disk
I followed this link for the partitioning, except,
In total, with the exact same windows 10 installation, we've tried this 5 times. We've dual booted two lenovas and one dell latitude and after entering the bitlocker key once after installation you'll never need to enter it again. However, we carried out the exact same steps with 2 dell xps models and each boot into windows requries the bitlocker key to be entered once again. This is not practical, but we're required to keep bitlocker enabled. The least worst solution we've found is using a usb key with the bitlocker key saved to it. This means that at each startup user requries the usb to be in the laptop. Awkward but less awkward than a 48 digit key...