If you cannot stick to these requirements, then either get tutoring or go on a group course (onsite or online), so that you make sure you prepare in time for the score you need.
It is important to do a diagnostic or sample test to check your starting level. You need to find out whether you require any additional basic preparation, e.g. improving your English skills, or refreshing the mathematical concepts, before you begin the focused test preparation. If you have the necessary knowledge, you can immediately focus only on the admission test. If your starting level is not sufficient yet, you should start with filling in the gaps. In this case, you might have to reschedule your test preparation because it will obviously take you longer. It is sensible to do a diagnostic test via a preparation centre because you can then get feedback from professional instructors.
Select prep materials and study plans
There is a huge choice of test preparation books, videos, online advice, sample questions and practise tests. Identify which of them cover the latest changes in the tests and which are most appropriate for your starting level and learning style. Also, be aware that some books and tools are meant to be used with an instructor, while others are perfect for self-preparation. Always research the reputation of the source which has produced the test preparation aids. Amongst other sources, always use the official publications of the test owners (GMAC for GMAT, ETS for TOEFL, British Council for IELTS, etc.).
Monitor your progress
When you are working on your own it is critical that you set clear targets and monitor your progress in order to ensure that you are getting closer to your goal. Although many students will be using sample admission tests from a variety of sources, it is important to select a single source and use it as a standard to check your progress. Also, be open to adapting your test preparation if you do not advance as planned. If needed, look for advice from professional instructors on how to proceed to meet your target on time.
Practising has different aims - to keep your concentration during the full test, to develop your sense of the timing and time management skills, and of course to master the different types of questions and be able to solve them correctly as quickly as possible. Practising is usually structured at the various levels as you advance with your preparation - you practise a single type of question, or a single section at a time, and finally, you practise full-length tests within the time limit.
It is also important to practise the format of admission test which you will be taking - for example, TOEFL has a paper and pencil format (PBT) and a test taken on a computer (iBT). Even if the test is only offered on a computer, you have to make sure you practise with the keyboard set-up which is used in the test centre. Also consider that during the test you will naturally be under additional stress. Practising helps you get fit for the test day and perform at your best. Often, especially with the GMAT, test-takers feel that they could have done better, so many of them retake the test to make sure they do the best they could have done.