1. Immersion Course
The idea behind an immersion course is simply to surround yourself by Spanish to such a degree that all your senses are saturated with it, in a similar way to how you learned to speak English as a kid.
The absolutely best way to do this is to go and live in a Spanish speaking country and insist on speaking nothing but Spanish. Ask people you are in regular contact with to correct your Spanish when you make mistakes and never be afraid of making those inevitable mistakes.
Your best option is to combine focused class work with speaking in real situations and staying with a host family who you will live with and interact with in Spanish. A good part of the class can focus on useful aspects of grammar – forget anything you won't actually use – and teach you how people really speak rather than how they are supposed to speak.
2. Intensive course
You can probably find an intensive course available where you live, in which you spend all day in class, 5 days a week, practising speaking Spanish in small groups or one-to-one with a native speaker and the dreaded grammar. Another option is to spend a couple of weeks in a Spanish speaking country doing the same and while it won't be as good as a full immersion you'll still see good results.
If you are in a Spanish speaking country you'll have the opportunity to speak Spanish at every opportunity you can find, although this may be more difficult where you live.
3. Regular Course
The standard Spanish course typically consists of one of two classes per week of a couple of hours at a time. These are frequently available wherever you live in the world as well as in Spanish speaking countries, so if you're unable to spend the time needed for an intensive course this could be a good option.
As always the focus should be on speaking and grammar restricted to what you need to know when you are holding a conversation.
4. Home study
In some respects this is the easiest option for many people, and in many others it is the hardest. It is easy because anyone with the course can spend time, preferably on a daily basis, to sit down and study Spanish. This can be on the train or in the car for the listening exercises.
However, much more discipline is needed to keep going and there are no easy solutions when you get stuck as you have no teacher. Also, your conversational skills will develop slower because you have nobody to hold a conversation with, your "conversations" will all be exercises.
Conclusion
As you can see no one method is right for everyone due to factors such as money, family commitments or simple lack of time. However, anyone who wants to learn Spanish should be able to find a course that suits their own lifestyle and time constraints. The key to learning is to be as effective as possible; that means whichever option you take you need to study the right things in order to make progress.