How to cast for a loved one: it's easier than casting for a stranger but it's still best if they cast for themselves or participate in some way. You need to be able to direct the spell, so being someone you know is easier than 'randomuser123 in America'. If you know how to cast spells you can cast for someone else, you should visualize the outcome happening to them, and a picture of the person, or a personal item can also help focus the energy.
Casting over someone unwilling: it's debatable. Whatever you view as morally right is the correct answer. Example, if this person is so sick they are sent to the hospital. If they don't want to be cast over, you need to determine if you should respect their wishes or help them. [Most Mormons are against blood transfusions from a religious view. Despite what the doctor thinks, they must respect the wishes of the patient] personally, in a life or death situation, yes, but to get your crush, no. Everything in between depends on the situation.
Even if this person doesn't believe in magick it could still work, but they will subconsciously be defending against the energy. It's the same if you pray for them, they're so cynical and against 'spirituality' for whatever reason they will not only subconsciously avoid the prayer but anything that might come as a result [say they pray to keep the person safe and they avoid being hit by a car] they will chalk up to coincide or luck. [Not saying it wasn't, but they will refuse to believe]
Without consent: easily, just cast it. People curse others, pray to protect their loved ones, and cast love spells over their crush all the time without first asking. It comes down to energy, focus, and intent. If you are casting a spell to protect someone who is unwilling, you will receive less negative energy in your direction [causing negative repercussions] than if you curse the person who cut you off in traffic.