This article explains what to do once you have added all your wallets and synced the transactions.
1. Go through your withdrawals and make sure they are actually withdrawals/payments and not transfers to your own wallets. If you find a withdrawal that is actually a transfer then you should add the destination wallet so Koinly can match them. If you can not add the destination wallet, you should mark the transaction as 'Ignored' so it doesnt impact your tax reports. Also tag any Gifted or Lost wallets. If you have sent some crypto as gift, you should tag the withdrawal as Gift. If you have transactions that were the result of a hack or theft; mark them as Lost.
2. Next, review the 'cost basis' (aka buying price / net worth) of each of your deposits. Koinly uses average market prices to automatically determine the estimated value of a deposit but this may not be what you actually had to pay to buy the asset. You should make sure the cost-basis is the same as your actual buying price. Also tag any airdrops, forks or other income.
3. If you see a 'Needs review' notice on the Transactions page then you will need to go through the highlighted transactions as well. The most common cause of such issues is missing deposits ex. you imported your data for 2018 but not for the previous years. In such cases Koinly does not have the correct wallet balances and if it encounters transactions where you are selling assets that you purchased in a previous year, it will show the 'Negative balances' message on the txn.