The first thing I check is the color of the balloons against the backdrop, because mismatched metallics or a banner that's a slightly different gold than everything else is the fastest way to make a party look thrown together at the last minute. Good birthday decorations hold a palette on purpose. If the napkins and the balloons and the banner letters all agree with each other, the whole room reads like someone planned it, even if someone planned it in twenty minutes on a Tuesday.
This stuff suits almost any home, which is part of the fun of it. A small apartment living room works fine for a cluster of balloons and one good banner over the couch. A family dining room can take a full tablescape with runners and confetti. Backyard parties in the warmer months want bunting and streamers that can handle a little wind, while an indoor winter birthday leans more on balloon garlands and lights since nothing is blowing around. The rule I follow is that the space dictates the scale, not the other way around.
Measuring up matters more than people expect. For a banner, measure the actual wall or window you're hanging it across, not just a guess, because a banner that's too narrow looks sad and stretched and one that's too wide bunches up at the ends. For balloon garlands, figure on roughly a foot of garland for every two to three feet you want to cover, and always buy a few extra balloons, because at least two will pop while you're tying them and that is simply how balloons work. The biggest mistake I see is going too small for a big room. A single sad banner on a wall in a large living room gets swallowed up. Oversize it a little, add a floor balloon or a second garland, and let the color do the filling in rather than leaving it sparse.
If someone has absolutely nothing to start with, I'd buy a balloon garland kit in a tight color story, maybe three shades of one color plus a metallic, and one bold banner that doesn't try to match it exactly. Everything else, the table runner, the confetti, the little paper fans, you can build up over a few birthdays without it ever feeling like you're starting from zero.
Does this color story hold together across the whole room, or is it three good ideas fighting each other?