how to fit restrictor in toilet pan inlet
Actually, let's start with some disambiguation, as we may be separated by a common language. I've been assuming you mean the lower portion of the toilet when you say "pan" - is that correct?

Given low detail and no picture in the question, I'll go on first principles and guess that a "toilet pan inlet restrictor" would involve removing the cistern/tank from the pan/bowl (generally a couple of screws or bolts, plus the plumbing) inserting the restrictor, and reinstalling the tank/cistern on the bowl/pan. And I'll second @bcworkz's comment that you should really make sure the outlet is not restricted - an anemic (restricted) flush into a bowl/pan with a restricted outlet is a recipe for a lot of clogs and flushes that fail to flush.

An easier version might install by simply lifting the flapper/flush valve and dropping it in place below the valve. But you may have an "outlet siphon" instead of a flapper valve, since your terminology appears to be British. That most likely does involve removing the cistern.