I have been learning English seriously in recent 3 years and my best IELTs score is 6 on average. That is not good enough to pass the minimal requirement for immigration, so I am still striving to improve my listening that is my most concern. 

My best part is speaking that has reached 7.0 on record 6 months ago. Depending upon my experience, there is no good means to improve English in a short period of time even some people claim you can make 0.5 grade progress within few months. My notion is unless you have good English foundation and you just lack exam skills, you cannot expect your grade to be lifted within few months. That is my case.

Hi Linh, here is a free course called Understanding IELTS offered by the British Council. https://www.futurelearn.com/courses/understanding-ielts/2/todo/2634

I've taken it. It's very helpful. The instructors will introduce the test and give many great advice to you.

I strongly recommend you to pay more attention on those useful links provided by the instructors at the end of each week. 

Good luck.

I almost concentrate all on my listening and reading, instead, only little time spent to keep my proficiency in speaking and writing.

@Allen: Thank you so much. That's my situation. I'm not sure about my experience but my English foundation is not bad. Speaking is the most difficult skill, so how could you gain such an excellent mark? And Did you prepare for IELTS test in 6 months for 4 skills or just writing and speaking?  

To Viet Nguyen

How long have you been learning English (Linh is still learning now)