To mass-convert leads in your Salesforce.com instance, you’ll first need to pull them using a SOQL query. To do this, click on Get Data button, and run a query similar to the following (you’ll probably need to add some of your own filters to pull only the Leads you need to convert):

You should get something similar to the following:

To convert some of the leads, select the rows containing them, and click on Convert Leads. To convert all leads in the spreadsheet – click on Convert All Leads. Don’t forget to set the values in the Lead Status column to a status that is set as ‘Converted’ in your Salesforce.com instance:

As a minimum, you’ll need to select which of the columns contains Lead Ids and Lead Statuses. Optionally, you can have a column of Account Ids and a column of Contact Ids in your data. Rows with Ids of existing Accounts and/or Contacts will be converted to the specified Accounts and/or Contacts, for rows without the Ids there – new Accounts and Contacts will be created. If you don’t have a column of Account and/or Contact Ids – they will be created for all records.

Click Go to convert all of the selected Leads. You’ll receive the conversion results on the right from your data, one column over: