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If you had chosen QuickBooks online from QuickBooks type drop down, We support OAUTH authentication even for QuickBooks online but you need to authenticate with additional security check from intuit. 

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You should key in QBO username and password to authenticate and connect with your QBO instance (OAUTH Authentication for QBO) and you will be prompted to enter an 6 digit code as an added security check from Intuit which will be sent to your registered email id.

Setup Processes 

In order to trigger the Integration, you will have to check the "Generate Customer & Invoice From Opportunity"  and click on save button.From the Setup Processes menu, User also has an option to upgrade his plan from the Free to either Standard or Enterprise depending on his integration needs. Before you actually pay for the plan, you You also have an option to take Free Trial of the plan you wish to subscribe.

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These are the few things which you have to keep in mind before initiating the integration using either Free or Standard version of Salesforce and QuickBooks. 

1. you You will have to change the page layout for the Accounts, Contacts, Opportunity, Opportunity product, Product in Salesforce which are is explained in the latter section.
2. If you are using QuickBooks desktop you can initiate the sync only from QuickBooks web-connector, Make sure to Setup QuickBooks web-connector by following the link.

If you are using QuickBooks online version, you can Initiate the sync from the Account\Product\Opportunity by the click of a DBSync button.
For Eg: When you want to Sync SF Account To QB online customer then hit the button "SF Account>> QBO Customer".  
 

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The below steps are explained for

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changing page layout of an Account in Salesforce and for the rest of the objects the same set of steps to be followed.

1. On the Salesforce Instance, Navigate to Salesforce Setup page by clicking on the Setup located on the top right section of the page and which is adjacent to Help menu.
2. From the Quick Search area located on the extreme left section of the page, Search for "page layout" and scroll down to find the Account/ pagelayout menu options to the left of the page.
3. Clicking on the Account\Pagelayout will open the Account page layout page, From there click on "Page Layout Assignment" and now click on Edit Assignment.
4. Select all the user roles for which you wish to change the pagelayout and from the option of Page Layout To Use , choose DBSync Account Layout and save the changes.
5. Now, Open any of the Account from the Account page where you will find the information as shown in the above image.