This would be done on the checkout page for the login link, the line of code looks like this by default:
<div class="text-indent">Is this the wrong account? Please <a href="users_LogIn.php">log in</a> or <a href="users_Registration.php">register</a>.</div>
If you add in the parameter you will be returned to that page after login like this:
<div class="text-indent">Is this the wrong account? Please <a href="users_LogIn.php?accessCheck=checkout.php">log in</a> or <a href="users_Registration.php">register</a>.</div>
This would take you back to the checkout page after logging in.