If I were in your shoes = If I were you = used when you want to tell someone what you would do in their situation.
When you make conditional sentences with 'if ' , you use the past tense to show that the situation is imaginary or unreal , and that you are not the other person . And, you cannot be.
This is even true for singular pronoun eg If he were to go there , he would hate it ( its just an imaginary situation . He never went)
To be = was or were .
But, You use only " were' to show an imaginined conditional sentence .