A closer look
Just like on the 350D and 750D, pushing on the top two corners of the front panel unlatches it from the rest of the case. Access is then granted to the (removable) front dust filter and to the screws that hold the intake fan in place.
The picture above shows the front panel hanging from its hinge, but don't worry: that panel can come off. If it didn't, getting to those fan screws would be needlessly difficult.
Here's a look at the left side of the 250D with the top and side panels removed. See that black box with the vertical stripes? The motherboard is supposed to lay flat on top of it, and the power supply, hard drives, and SSDs are meant to go underneath. The area between that box and the front intake fan is reserved for cable routing. Mercifully, Corsair protects the fan blades from stray cables with a removable grill.
Here's the view from the right. We can't see much of note from this angle, except perhaps for the two side 120-mm fan mounts (one of which is populated by a Corsair AF120L) and the sides of the 2.5" and 3.5" drive bays. Those bays aren't populated from the inside, however.
They're populated from the back.
The 2.5" and 3.5" drive trays face the rear of the case, and they're hidden behind a small grill, itself held in place with thumbscrews. There are two trays for each drive size. The 3.5" trays are similar to those in other Corsair enclosures: they have a flexible plastic frame with steel studs designed to mate with a drive's screw holes. The 2.5" trays follow the same basic design—which, incidentally, is a first among the Corsair cases I've seen. The 350D and 750D both use other kinds of brackets and contraptions to fit 2.5" drives.
Now, the 250D isn't really limited to two SSDs. Its two 3.5" trays both have 2.5" mounting holes in their bellies, so if you don't mind sacrificing mechanical storage, it's possible to have four SSDs in all. You'll just have to screw the last two into place manually.