When I saw this photo pop into my Pinterest feed this morning, it immediately brought back memories of an old home office from a previous apartment. Not because my home workspace looked anything like Aly's from design blog, A Little Tipsy, but for the fact I recognized she had made a very similar giant-sized mousepad I had years back...
All it really takes is the IKEA PRÖJS desk pad and some decorative paper (or really anything you want to put underneath for your pattern or image). After lightly spraying the back of the desk pad with spray adhesive (be conservative, you don't want clumps of glue), place your sheet and carefully cut using an X-acto blade (Ally uses scissors, but I found using an X-acto blade is much more precise).
A few more photos of Aly's DIY project over at A Little Tipsy.
Here's what my pad looked like from a few years back (oh, I remember that Dell 22" screen fondly) from a former home office.
An additional note: several months after I completed my own DIY mousepad using the same technique as Aly, I noticed the clear IKEA desk pad started exhibit some waving due to the heat of a Los Angeles summer, the fairly thin plastic was curling at the edges. You may want to consider adhering a back panel of some sort to prevent this from happening. Cork adhesive drawer liner is easy to cute and should provide both some support to prevent curling and also the addition of a softer mouse pad feel underneath.
(Images: A Little Tipsy; Gregory Han)