Write them the same way you'd write any other trigger. If you click onto the "Validation Trigger" button (it appears only on report parameters properties list), you'll see that it offers you to write a function that returns a Boolean (true or false).
Body of the function depends on the parameter. For example, if you expect that user should enter a number less than 5, trigger would be something like
if :par_value > 5 then
In that way, if user enters (let's say) 7, trigger will fail and the user will be prompted that the parameter input was invalid.