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You can't. You can't describe pain. You can't describe what things taste like. You can't describe much at all about our physical sensations of the world. Language just does not seem to work like that. There are no adequate words for any of it. 

What you can do is evoke the memory of sensations that the reader has felt. You may also be able to provoke them to intensify the memory (maybe). You do this by writing about things that evoke those feelings and sensations in memory. Talk about holding your hand over a candle flame and anyone who has ever burned themselves will have some memory of that that feels like. 

But be cautious here. Your reader may not actually be looking for your prose to create or recall the sensations of pain from their past. Pain is ickey. People tend to read so they can have dangerous adventures without the ickeyness of actual painful and unpleasant situations. 

This does not mean that they don't want to read about events that involve pain and suffering, but don't assume that they want to imagine themselves suffering that pain. More likely they want to observe your characters suffering without experiencing it themselves. So think about what it is like to know that a friend (or enemy) is in pain. Try to evoke a memory of what **that** feels like.