How to combine CSS and javascript files?

I ran a test today and I one of the suggested improvements is to combine CSS and JavaScript files. So how can I do this while ensuring that everything works? There are 15 JavaScript files served from bosmancommunicatie.nl. They should be combined into as few files as possible. https://bosmancommunicatie.nl/wp-content/plugins/add-to-any/addtoany.min.js?ver=1.0 https://bosmancommunicatie.nl/wp-content/plugins/cookie-bar/js/cookie-bar.js?ver=1536323235 https://bosmancommunicatie.nl/wp-content/plugin ... or-wordpress/assets/js/frontend.min.js?ver=7.1.0 https://bosmancommunicatie.nl/wp-content/plugins/jetpack/modules/wpgroho.js?ver=4.9.8 https://bosmancommunicatie.nl/wp-content/themes ... ive-pro/js/catchresponsive-custom-scripts.min.js https://bosmancommunicatie.nl/wp-content/themes ... /js/catchresponsive-scrollup.min.js?ver=20072014 https://bosmancommunicatie.nl/wp-content/themes/catch-responsive-pro/js/fitvids.min.js?ver=1.1 https://bosmancommunicatie.nl/wp-content/themes ... o/js/jquery.cycle/jquery.cycle2.min.js?ver=2.1.5 https://bosmancommunicatie.nl/wp-content/themes ... responsive-pro/js/jquery.sidr.min.js?ver=2.2.1.1 https://bosmancommunicatie.nl/wp-content/themes ... responsive-pro/js/navigation.min.js?ver=20120206 https://bosmancommunicatie.nl/wp-content/themes ... e-pro/js/skip-link-focus-fix.min.js?ver=20130115 https://bosmancommunicatie.nl/wp-includes/js/jquery/jquery-migrate.min.js?ver=1.4.1 https://bosmancommunicatie.nl/wp-includes/js/jquery/jquery.js?ver=1.12.4 https://bosmancommunicatie.nl/wp-includes/js/wp-embed.min.js?ver=4.9.8 https://bosmancommunicatie.nl/wp-includes/js/wp-emoji-release.min.js?ver=4.9.8 F0 Combine external CSS There are 15 CSS files served from bosmancommunicatie.nl. They should be combined into as few files as possible. https://bosmancommunicatie.nl/wp-content/plugins/add-to-any/addtoany.min.css?ver=1.14 https://bosmancommunicatie.nl/wp-content/plugins/cookie-bar/css/cookie-bar.css?ver=4.9.8 https://bosmancommunicatie.nl/wp-content/plugin ... e-buttons/public/css/button-styles.css?ver=2.1.1 https://bosmancommunicatie.nl/wp-content/plugin ... hortcode-buttons/public/css/font-awesome.min.css https://bosmancommunicatie.nl/wp-content/plugin ... inc/genericons/genericons/genericons.css?ver=3.1 https://bosmancommunicatie.nl/wp-content/plugins/jetpack/css/jetpack.css?ver=6.2.1 https://bosmancommunicatie.nl/wp-content/plugin ... dgets/social-icons/social-icons.css?ver=20170506 https://bosmancommunicatie.nl/wp-content/plugins/tablepress/css/default.min.css?ver=1.9 https://bosmancommunicatie.nl/wp-content/plugin ... blic/assets/css/vfb-style.min.css?ver=2015.11.29 https://bosmancommunicatie.nl/wp-content/plugin ... s/css/vfb-display-entries.min.css?ver=2017.01.30 https://bosmancommunicatie.nl/wp-content/plugin ... c/assets/css/vfb-payments.min.css?ver=2015.01.16 https://bosmancommunicatie.nl/wp-content/themes/catch-responsive-pro-child/style.css?ver=1.0.0 https://bosmancommunicatie.nl/wp-content/themes ... sive-pro/css/jquery.sidr.light.min.css?ver=2.1.0 https://bosmancommunicatie.nl/wp-content/themes/catch-responsive-pro/style.css?ver=4.9.8 https://bosmancommunicatie.nl/wp-includes/css/dashicons.min.css There are 2 CSS files served from secure.gravatar.com. They should be combined into as few files as possible. https://secure.gravatar.com/dist/css/hovercard.min.css?ver=2018Sepaa https://secure.gravatar.com/dist/css/services.min.css?ver=2018Sepaa