There is two way to get connected.
1- At QuickBooks App Store search for Online Check Writer
Click on “Get App Now” to authorize QuickBooks to integrate with OnlineCheckWriter.Com.
At Online Check Writer, add all the Bank Accounts. (Setting > Check Design>Add NEW)
Make check payments as you normally enter them in QuickBooks Online. OnlineCheckWriter automatically picks it up, and it will be ready to print or mail it to your customers/vendor/clients at OnlineCheckWriter.Com instantly. If you select mail it, OnlineCheckWriter.Com print that specific checks stuff in envelopes and mail to your payee destination for free from our subscribed customers. You pay the cost of the stamp, envelop, and check paper. Usually, it comes below one dollar if you select first class USPS mail.
2. If you already have an account with onlinecheckwriter, go to setting after login to online check writer and click on QuickBooks connect. Select QuickBooks Online and integrate. It prompts you to enter QuickBooks’ username & password and your permission to communicate with each other.