Solution 1 : Unofficial plugin
This is the easiest way but in this case ~5% of yours earnings are donated to the developer of the plugin.
You can find the unofficial plugin on wordpress.com
Solution 2 : Manual installation
If you prefer to keep all your earnings, you will need the Header and Footer plugin to inject Coinhive's script
Once the plugin installed, just inject Coinhive's script in the header like the following, according to the documentation, don't forget to replace the script with you own site key, and with your own configuration (throttle, threads ect..) :
Happy mining !