Note - CryptoTrader.Tax does not currently support the taxation treatment of margin trades.
Select "My Trade History & Analysis" from the Orders drop down
Select "Export History"
Select the "Trade" option and export your trade history from ALL years of trading using Poloniex's date ranges.
(You many have to export multiple files to capture all of your history. Please make sure dates do not overlap to avoid duplicate trades).
DO NOT open the file in Excel and "save as" a new file.
It's important to upload the raw file that Poloniex exports straight from your "Downloads" folder. This avoids rounding errors and corrupting the file. If you need to edit your CSV, please use Google Sheets.
Upload your trade history files into the Poloniex tab on Step 2 (Import Trades).