You first need to block out some time. Somehow, papers do not grade themselves without some time devoted to them.
Get all the papers or tests you have to grade together, in one place. A folder or envelope helps.
You need a physical space in which to grade. It doesn't matter where, but papers need to graded in some specific place.
Some substances might come in handy. Caffeine or small amounts of alcohol. Some anger managements techniques might also be necessary.
I have bad handwriting and fast typing, so I type out my comments on the computer after correcting some grammar mistakes and putting some ?? and WTF and !! on them. (Usually I don't write WTF.) When I am done, I just insert page breaks between each student's comments and print the whole thing out, stapling or paper-clipping the sheet to the student's paper. That way I also have a computer file with all my comments and grades, so I can see what problems particular students have between one paper and the next.
Estimate how many papers you can do in an hour. For example, I could do one 20-page graduate paper in an hour, or 6 four-page undergraduate papers in the same time. Don't do the good ones first and save the bad for last. Rather, work more or less at random. Use the best ones to set the standard for what an A should be, and put papers in the order their rank, good to bad. Make sure a worse paper does not have a better grade.
I work one hour on, one hour off. So this evening, for example, I graded 6 paper between 7 and 8 p.m., then six more between 9 and 10. That leaves five more for the morning between 8 and 9.
I hate grading, but I also manage to be quick if I want to be.
If a student turns in a paper late, they do not get it back with the rest of the class, even if I have it graded already. I view the grading of a single batch of papers as a single task, and keep all the papers physically in one place until they are graded.