When you install JooCart WP ,it installs OpenCart automatically.
So no need to install OpenCart separately.
But if you want to connect JooCart WP with existing opencart website,you can do this in three different ways.
First Method:
1. Go to WordPress admin Left Side Menu->Settings->JooCart WP Options
2. Provide there "OpenCart Directory Path" and "OpenCart Directory URL" and save. It will connect your existing OpenCart with WordPress.
Second Method:
Export products from existing OpenCart site to JooCart WP.See this How can i export OpenCart products from my another site to JooCart WP?
Third Method:
1.Go to WordPress Menu Settings->JooCart WP Options.
2.Set No for "Use WordPress Database".
3.Put values for Database Username ,Database Password ,Host ,Database Name ,Database Tables Prefix of Existing OpenCart site.
4.For digital product's downloads,products images,new extensions etc ,upload them from OpenCart directory to JooCart WP directory(wp-content\plugins\joocart\). Also enable vqmod if your new extension need vqmod as follows How to install vQMod in JooCart WP
5.Done
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