Two fields, one switch, two buttons. That is the whole thing.
Where to go
In your WordPress admin, click Mollie Payments in the left menu, then open the API keys tab.
Paste in your keys
You will see a section called API keys and mode, with two fields.
Live API key. Paste your live key here.
Test API key. Paste your test key here.
You can fill in both now, even if you are only going to use the test one for the moment. That way everything is ready when you go live, and you will not be hunting through the Mollie dashboard at the exact moment you want to start selling.
Choose your mode
Above the fields is a switch called Active mode, with two settings: TEST and LIVE.
This is the switch that decides whether real money moves. Only one is active at a time.
Set it to TEST while you are setting up.
Click Save.
Test the connection
Now click Test connection.
Please do not skip this. It takes two seconds, and it is the difference between finding out now that your key is wrong, and finding out in an hour when you cannot work out why your checkout is empty.
The plugin asks Mollie directly whether the key works. Green means you are connected.
If it fails, it is one of three things: you copied the key incompletely, you pasted the live key into the test field or the other way round, or your host is blocking the plugin from reaching Mollie.
Test the webhook too
While you are on this tab, scroll down to the Webhook URL, and click Test webhook.
We give this its own article, because it matters more than it looks. But do it now: if that button fails, your payments will succeed at Mollie and do nothing on your website, and that is the most confusing problem you can have.
What happens next
Look at the badge at the top of the plugin. It now tells you exactly where you stand, and the next article explains what each state means.