So to make the slouchy beret, I used needles that are one size bigger (5.0mm from the original 4.0mm) and I did 19 more repeat rows in stockinette stitch after the last increase. Meaning to say that After the last increase row I didn't increase anymore, but instead did 19 more rows of stockinette stitch before I move on to the decreasing rows.
The result was a jumbo sized beret. I think I should have started off with smaller needles (I used 4.0mm) for the ribbings in the beginning. Fortunately, my beret was knitted flat, so I simply stitched up more of the seams again and again after I achieved the desired result.

(Kelly is wearing the beret in the main picture. I gave this beret to her)