Hi, I am Dave, I will help you with this.
Right click your Start Button and open Disk Management
Right click the C drive partition and choose to Shrink that partition.
Shrink it the amount you want the D drive partition to be, on your drive size, ensure the C drive partition is left at about 120GB so Windows has plenty oof room
When shrinking the C drive partition, that is calculated in MB, so 1000 is 1GB
Then when you have shrunk the C drive partition, right click the resulting 'Unallocated Space' and select 'New - Simple Volume' and accept the defaults.