Hi Saheed,
Start writing now
Seriously - if you write your choices document
as you are working on your assignment then you will find the job much easier than trying to remember what choices you made and why you made them at the end of the project.
For example, you might choose today to use a RandomAccessFile instead of DataInputStream and DataOutputStream classes. But if you wait until you have completed the assignment you might have forgotten that you even considered using the DataInputStream and DataOutputStream classes; or if you do remember that you had those choices, you might forget why you chose them.
As for tense and grammer - if they are worrying you, write your document as bullet points. Something like:
JTable instead of JTreeSpecified in instructions Is going to take up far less space, and is going to be easier to write than "
I chose to use a JTable instead of a JTree because the instructions said that I must use a JTable, and passing is something that I consider worthwhile"
(Sorry for the silly choice example by the way
)
Oh - and if it matters, you can see that I have used past tense - from the perspective of the assessor, you will have completed your assignment, so the choices will have happened in the past.
Regards, Andrew