The thing to know before you buy relaxed jeans is that relaxed doesn't mean baggy and it doesn't mean shapeless, it means there's room through the thigh and a straighter line down to the ankle, so you want to try them sitting down in the changing room, not just standing in front of the mirror looking pleased with yourself. If they feel fine standing but pull tight the second you bend your knee, that's not a style you're reading wrong, that's just the wrong pair.
They're not the ones to reach for if you want your legs to look longer or your waist to look smaller, because that's not the job they're doing. Anything with a lot of stretch in the denim tends to lose the shape by lunchtime and go a bit saggy at the knee, so I'd look for something with a decent cotton content, maybe five percent elastane at most, so it keeps its line. A rigid, slightly stiffer denim actually softens into a better shape over a few wears than anything too soft to begin with. They're brilliant for a day where you're walking a lot, sitting on trains, picking things up off the floor, the ordinary physical business of a Saturday, but I wouldn't wear them anywhere I needed to look pulled together in ten minutes flat, because they take a bit more styling than a skinny jean does.
Where they really earn a place is autumn through to early spring, with boots and a jumper that's got some weight to it, or in high summer with nothing on your legs and a flat sandal, when the straighter cut stops your ankles looking swamped. What they don't get on with is anything else oversized up top. A big jumper over a relaxed leg just reads as a lot of fabric and not much shape, and you end up looking like you've borrowed the whole outfit from someone taller. Tuck something in, or at least go for a top that ends at the hip, so there's a line somewhere on you. I'd also steer clear of chunky trainers with a very wide sole underneath them, it tips the whole thing into clumpy rather than easy, and easy is the entire point of the cut.
Before you commit, sit down in them properly, do the squat test by the mirror even if it feels daft, and ask yourself whether there's room through the thigh without looking like you've borrowed your brother's jeans by accident.