Keep hitting your backside with it . Either by your partner or self spanking .
According to my books on the subject ( I had to get this to settle my psyche when my Mrs wanted me to beat her on the backside) your skin will eventually harden and they use the term leatherback .
I don't know how long this will take but I am guessing it will be over a period of time . Perhaps in the mean time if you wanted to endure more then perhaps over clothes for now then remove layers as the process continues.
What book suggested this? I'm genuinely curious, not criticizing you, because from everything I have read on the subject this is something you DON'T want to happen, not something you should try to make happen. Hence why people who recieve spankings regularly actively take steps to look after their skin and prevent this happeing with lotions and such.
I'd definitely go with the softer side of the paddle if you can't take the pain rather than trying to harden yourself pyhsically to be able to take it. The feeling of 'too much' could well be coming from more of an emotional place than a physical one, and you don't want to push that.
+1 - I have had lots of spankings from Mr Scorpius using various toys, from paddles to floggers, his hand etc. and my skin is still very soft. I do look after my skin, and always moisturise after a heavy session with cream, arnica or aloe depending on how intense it was. You really have to just take your time, and build up at your own level until you feel comfortable. If something is painful or too much, then just slow down - never push past your boundaries. It may be that at the moment this is your limit, and that is fine, just listen to your body and go at the pace you feel comfortable with :) xx