One option is to organize your essay from general to particular . For example, if you were describing the new Big Tex at the State Fair of Texas, you might start out by describing the setting - the smell of the corn dogs frying, the crowds of people, the happy children dodging in and out of the midway games - then get into his grandiose size - after all, everything is bigger in Texas, right? After that, you describe the details: his new crisp, white, pearl snap shirt, his blue jeans tucked into cowboy boots, adorned up top with his signature belt buckle, and so on.
It really depends on a number of factors, one of which is your understanding of the test and requirements of the band scores. If your English is suitable for the band score you are aiming for, you will only need to focus on exam skills to make sure you reach your potential. The skills will help you understand how to approach questions and what the examiner is looking for in your writing and speaking. Once you feel comfortable with all that, you will have a clear idea of how long you want to practice before your test. Whatever you decide, make sure you are frequently getting the band score you want in practice tests.
All the best