Depends on what you mean by front-end development. If its HTML/CSS and some javascript to make it work - the best way to learn is probably to just find designs

Depends on what you mean by front-end development. If its HTML/CSS and some javascript to make it work - the best way to learn is probably to just find designs for websites/apps on a site like dribbble and try to hack it together. Try codepend.io as well - looking at how others accomplished something styling you might be struggling with also works well. If you're talking about the more modern description of front-end - which might mean ReactJS, VueJS, or any of the other hundreds of modern frameworks - I'd say follow some tutorials on YouTube to get the basics and then just practice building simple apps. For someone who is a backend developer and already knows how to code, it shouldn't be too difficult. You'll probably have a harder time learning CSS and understanding good design and UX.