The mattress showed up compressed to 38 inches long and about 42 pounds. I remember that number because I almost threw my back out carrying it up the stairs. The box wasn't huge, but it was dense. So if you're waiting on a soft memory foam mattress, plan for that weight. And plan for the biggest decision you'll make in the first week: the foundation. Not the fancy headboard, not the sheets. The surface under the mattress. That's it.
If you put a soft memory foam mattress on a bent slatted frame with gaps wider than three inches, you'll feel every slat through the foam. I've seen it happen. The mattress sags, you toss all night, and you blame the mattress. It's not the mattress. It's the base. So before it even arrives, check your bed frame or box spring. Is it solid? Are the slats flat and less than three inches apart? If not, get a piece of plywood or a bunkie board. Cheap fix, and it makes the difference between a mattress that feels like a cloud and one that feels like a bad motel bed.
Day one setup is boring but important. Put the rolled mattress on the foundation, cut the plastic carefully (don't slice into the foam — I've done that, it's a scar that never goes away), then let it expand. For a soft memory foam, that can take 24 to 48 hours. My first one was still puffing up on day two, especially at the corners. Don't sleep on it until it's fully expanded. That's tempting, but you'll get dips that never fully recover. I know, it's the first night, you're excited. But wait. It's not a race.
What else? Smell. There will be a smell. Memory foam off-gassing is real. I had to open windows and run a fan for a couple of days. It's not pleasant, but it fades. If it doesn't fade after a week or gets really bad, that's a return. But most of the time it's just the foam settling.
Now, the one decision: firmness. Wait, you bought a soft memory foam mattress. You already decided that. But here's the thing—within 'soft', there are two versions. There's soft in the top layer, and then there's the base support foam. That's the one that matters. A high-density base foam (like 1.8 pounds per cubic foot or more) will support you and hold up over years. A low-density base (think 1.5 or less) will sag and soften even more in a year. So when you're reading specs, ignore the top layer claims initially. Look at the base foam density. That one number decides whether your soft mattress stays soft but supportive, or turns into a hammock. I've had a cheap one and a decent one side by side. The cheap one felt plush for two months. Now it's a crater.
Another thing: rotating. Yes, memory foam doesn't flip, but rotating 180 degrees every three months helps even out the soft spots, especially in the first year when the foam is still breaking in. Doesn't hurt. Also, if you use a heated mattress pad, be careful. Some memory foams don't like heat, it can accelerate softening. I'd wait a week before using anything warm.
And one more little tip: give it a few nights before you judge the comfort. The foam adjusts to your body temperature and weight. That means your first night might feel firmer (if the room is cool) or softer (if the room is warm). My soft memory foam mattress felt pretty firm on night one because my bedroom is cold. By night four, it felt like a different mattress. So don't rage-return after night one. Give it at least a week, maybe two. You'll be sleeping on something that's still changing.
Setting up a soft memory foam mattress is mostly waiting and checking a few things. The base, the base foam density, and time. Those are the only things that matter. I know you're tempted to just lay it on the floor. I did that once. It's okay temporarily, but the floor doesn't allow air circulation underneath and the mattress can trap moisture. Better to have a proper slatted frame or a box spring.
If you're diving into other memory foam adventures, there's What Everyone Gets Wrong About Memory Foam Mattress Prices and Serta Memory Foam Mattress: First Week Setup and the One Decision if you went with a Serta. And if you're thinking about a budget pick, check A Plain-English Look at Twin Mattress Under $100 — the same rules apply.
Now, the one thing to do today: go measure your bed frame slats. Grab a tape measure, stick it between the slats, and check that gap is under three inches. If it's not, order a bunkie board before the mattress arrives. Twenty minutes spent now saves you a week of bad sleep.