Rationally. Not impulsively/instinctively.
Loving is a rational decision. The fairy tale stories of happily-ever-after and the wild Hollywood passions are ludicrous representations of what love actually is. Assuming you're looking for long-lasting, committed romantic love, that is. Because love takes many other forms: an exciting summer fling that keeps a sweet spot in your heart years later; a deeply committed friendship with someone you care so much about that you cry for their pain; the bond between siblings, cousins, or parents and children which makes us argue and shout and fight about nonsense but also makes us give kidneys and bone marrow without a moment's hesitation. Love comes in so many forms that the very question how should you love someone? rings with pointlessness. (Sorry.) But if what you mean is how should you love someone romantically and how do I know when to do it? Then that is when I offer my method: Be rational. Not impulsive.
Don't try to love someone who isn't available to love you. Whatever that means in your situation.
Don't try to love someone who doesn't want to be loved by you (or by anyone). You'll know that when you see it.
Don't try to love someone who takes advantage of your softness. They can challenge it, but they can not destroy it.
Don't try to love someone who doesn't respect others. Especially someone who says bad things about others to you.
Don't try to love someone who doesn't keep in touch. It's 20fucking19. There's no excuse for leaving someone hanging.
And... don't try to love someone who just doesn't feel the same. It doesn't mean they don't care. Loving is a big choice; allow people the freedom to make it as they see fit, and if someone is not ready for that choice you have to accept it and wish them well.
Do love someone who you find yourself at-ease with. Someone who, although they may be thrilling at the beginning, feel like home.
Do love someone who listens to you, meets you where you are, and respects your needs.
Do love someone who respects and loves themselves. This looks like honesty and health.
Do love the person who doesn't make you wonder if you've found the right person, and who doesn't make you wonder if you're doing it right.
Until then, trial and error it, baby! Sometimes it hurts, but learning is the only way forward!
And remember, love is an ongoing decision. You don't make it once. You choose to show up and love every single day. And make sure you're loving yourself that way, too.