In my opinion thinking in a foreign language is obviously possible. It may be considered one of the learning methods, but it has it's disadvantages. The quality of your 'thinking' depends on your level. When you are advanced, you know a lot of vocabulary and the most frequent grammar constructions - you may think about everything and your thoughts will be like the fluent speech (or at least you will imagine it is). When you are beginner, you may try to think using the vocabulary and grammar you already know. In fact it may help you with talking to foreigners.

How to start? Imagine that you meet a friend who can speak only the language you learn. This person is with you all the day (in your mind of course) and you have to explain him everything you do. If something just came to your mind - share it with him in your target language. And remember - communication above all. When you start thinking - just verbalize it at once and don't wait until you will know what to say. If you have idea that it would be better to create the sentence and then translate it - don't do it, drop it, try to say it in shorter sentences with easier vocabulary. When you think you need a dictionary - forget it and try to find other word (maybe a few). Your imaginary friend will not wait.

Of course you will make enormous number of mistakes that way. You may check them later and discuss with your teacher. Your aim is to adapt your brain to verbalize your thoughts in other language at once. It helps when you meet real person and have to talk to her - you speak faster though not perfectly correct.

Will you speak fluently after this exercise? Unfortunately not. Our brain plays tricks on us, makes shortcuts in our thoughts - we may think we have built the complete sentence... and skip it or substitute with our native language. So when we try to speak we recognize we are not so good as we thought. So it is not the method you can use separately.