At first, you need to enter all your current balances (Deposit/Income) from all exchanges (for every Coin and with the exchange names)
Then you can set up the API jobs for the supported exchanges and set a start date of today.
This way all future transactions will be imported into the system.
Please keep in mind, that you still need to add missing transaction types, if the exchange does not gives them out in the exports.
Most of the time the missing transaction types are deposits and withdrawals.
You could bundle them and only enter the difference to your current holdings with a "General Deposit or Withdrawal"
Since you only need to know the insights of your trading activities, you will get them for all future transactions.
What is important, is that after the whole process you should not rely on the "Realized and Unrealized Gains" page,
because it is based on a tax compliant calculation and it's going to be misleading without all the historical data.
You should use the "Average Purchase Prices" page instead.
All other analyses are going to work as expected.