Is there a way to purposely shrink clothes?

I don't know what was wrong with me that day, but I bought 2 packs on knickers in size 16-18 and I'm an 8. I want to shrink them down cause they're actually like shorts at the moment.

Is there a way to, on hot wash or something? x
Update: @Katiemw2010 Mine wer from Primark too... thats probably what I did! :) x
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