Open and Login into Think or Swim. 

Go to the Account Page and then click on the hamburger menu on the top right. 

Click on Export to File and Save it to your Desktop.

Once you save that file, go to https://www.tradeportfoliomanager.com/Members/Trades/Import or login to www.tradeportfoliomanager.com and go to Import 

Once you click on Import click on your account that you want to import and then select Think or Swim (Paper Trading)

Click on Browse to find the file you just saved.

Click on Import Trades.