The easiest way to set up a one-way link is to type it directly into the destination cell. For example, if A1 contains 123, type the formula
=A1
in any other cell, say E10. The contents of E10 will always be the same as A1. This is identical to a standard spreadsheet formula, so =A1 + 10 is also valid.
As an alternative, you can use a paste special operation to achieve the same result. For example:
Click in cell A1
Select Copy by right-clicking your mouse
Click in cell E10, which is to contain the linking expression
Select Paste Special by right-clicking your mouse
Select Paste Link from the "Paste Special" dialog
See also: