You may need to remove a PSU mounting plate depending on your case, but most will require you to push the PSU into the rear of the case, from the inside.
Step 1: Most cases are designed for the PSU to be installed with the fan facing down, letting it pull cool air from outside the case, but check your manual if you're unsure. You may need to remove a PSU mounting plate depending on your case, but most will require you to push the PSU into the rear of the case, from the inside.