Just sending the file may not really help much, if you dont know how to do it

1. right-click on any cell and select Define Name
2. in Name, enter this...
ContractItem
3. in Refers To, enter this...
=OFFSET('Data input'!$C$2,0,0,COUNTIF('Data input'!C:C,">""")-1,1)

You now have a dynamic named range.

4. Wherever you want the Iten DD, enter...
- click data tab/data Validation
- under Allow, select List
- in Source enter...
=ContractItem

If that still doesnt help, explain which part you are struggling with