Every Sunday, I wage a war on my fitted sheet as I attempt to put it on my mattress, struggling to fit it around one corner as it pops off another.
On a recent Reddit thread, u/ericfussell had a brilliant, simple tip if you struggle like I do: After you've successful covered your first corner, move to the diagonal corner for your second attempt. Fitting the diagonal corner isn't going to cause the first one to pop free.
According to Peter Cancelli, founder of MattressExpert.com, this advice works especially well when dealing with sheets that might be a little too small for your mattress (and require more effort).
Look for the fitted sheet's tag, too. The corner with the tag is often meant to fit over the bottom right side of the mattress if you're facing the bed. From here, you can find its diagonal corner and fit the sheet on both ends.