On your PC, press Windows + R at the same time. The run dialog will open.
Type in: diskmgmt.msc. Press enter.

Now, find your phone's disk in Disk Management. It will be a disk with many partitions (parts).
Find the Data partition, then right click on it. Select Shrink Volume...

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Enter the space you wish for your Windows 10 ARM partition. I suggests at least 6 GB minimum, 8 GB for more space.

Choose Shrink.

• If it say something like Cannot shrink a partition containing errors., open Command prompt as Administrator.
  Type this: Chkdsk /f <MainOS>:\Data
  Where <MainOS> is your MainOS's drive letter. Wait for the command to finish and retry the above steps.

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Right click the new Unallocated block, select New Simple Volume. A new dialog will open.
Next, Next, then you meet this screen:

Choose Next.

File system must be NTFS. Tick Perform a quick format if unticked.
Then Next, Finish.

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At this point, you are ready to move on to the next part.