An interesting question that depends on what makes them feel loved.

Be there…. Always be by their side.

Seriously. Just be there. When they need you or want you to be there. I mean emotionally, mentally, and physically.

Someone being terrible to them? Call that person out. They trying some new hobby? Show interest and support, even if you aren't into it. Are they struggling with something on a personal level? Tell them they are not alone and you're gonna be there to help while they get through it.

Nothing worse than calling on the person you believe loves more than anyone else in the world and having them tell you they're too busy for you, or having them there and constantly distracted by other things, or not seeming to really care about what's going on that you needed them there so badly.

Along the answers of what others have said though, it's a bit different for everyone. I think the real answer is “learn their love language.” Everyone accepts and gives loves differently. I'm a quality time kind of person so you being there for me tells me a lot. It also means that being there with you and for you is the way I best show my love.