Make sure you know the material on the syllabus (
http://www.cs.ox.ac.uk/admissions/undergraduate/how_to_apply/syllabus_2018.pdf) thoroughly.
Read the instructions for candidates (
http://www.cs.ox.ac.uk/admissions/undergraduate/how_to_apply/candidates.pdf).
Then look at past papers (
http://www.cs.ox.ac.uk/admissions/undergraduate/how_to_apply/MAT_test.html#Specimen_and_past_tests): attempt the questions, and then compare your answers to the model answers. At first, don't worry too much about speed: spend as much time on each question as you want. The questions are difficult, and so you need to get used to the style. As you do more past papers, you'll get faster at them. Towards the date of the MAT itself, start doing past papers to time. Also, attempt the questions on a print-out of the paper itself, so you get used to using the space available (there are blank pages at the end).
In the longer questions, there is often a progression of ideas through the different parts. Be on the lookout for where you can use the results of earlier parts in later parts.
Give enough explanation to persuade the marker that you understand. Alternatively, give enough explanation that would help one of your colleagues to understand. Partial marks will be given for partial explanations.
In the multiple choice questions, if all else fails, guess: there is no penalty for incorrect answers. Better is if you can eliminate some of the options, and then guess from the remainder.
Think about which questions you find easier. You might want to attempt them first, to build your confidence, and to make best use of the time available.
Gavin