???? If you live in the USA that return was due LAST week....

Well, take it in pieces. Get a box and everything that is NOT connected to the task, you put into that box. That will help clear the area so you can work.

Just do one thing at a time, and tell yourself that's all you will do right now... get the box. put odd things in it (don't sort them now...don't read through them or worry etc... unless they are connected to your urgent project, put them in.)

gather what you will need to do the task. if that includes records, can you pull the whole file of stuff or all the paperwork where it's at... and sort it on the next piece of attention/time?

Sit and organize your work: reference materials, paper, pen, pencils erasers white out lol etc

Then decide according to how you best work, do you take it one line item at a time, or would you do better to pull all the background material from the rest all at once...

Decide ahead of time how long you will spend on each task before leaving it and coming back to it later.

Sometimes just making decisions about how to do it gives the impetus to finish!