As the sibling comment mentions there are several aspects.
You'll need proper input filtering which may require a non-trivial filter network. You'll also need proper output filtering, which does include slapping a lot of capacitors on there, but also careful selection of those capacitors both type and size. Parasitic inductance of larger packages can mean they can't filter high frequencies, and MLCC capacitors have a DC bias which means the effective capacitance is significantly reduced when they have a DC bias on them which they will have in a DC-DC converter.
Then you need to take great care about component placement and board layout, to minimize the return path of the currents and such.
You can skip all of that and get a board that functions as a DC-DC converter if you measure it with a multimeter, but actually be horrible. And you just can't fix bad layout by slapping more capacitors on there. And even with a not terrible layout, you can't fix it by using the wrong kind of capacitors. Like anything through-hole is just not gonna pass.
You'll need proper input filtering which may require a non-trivial filter network. You'll also need proper output filtering, which does include slapping a lot of capacitors on there, but also careful selection of those capacitors both type and size. Parasitic inductance of larger packages can mean they can't filter high frequencies, and MLCC capacitors have a DC bias which means the effective capacitance is significantly reduced when they have a DC bias on them which they will have in a DC-DC converter.
Then you need to take great care about component placement and board layout, to minimize the return path of the currents and such.
You can skip all of that and get a board that functions as a DC-DC converter if you measure it with a multimeter, but actually be horrible. And you just can't fix bad layout by slapping more capacitors on there. And even with a not terrible layout, you can't fix it by using the wrong kind of capacitors. Like anything through-hole is just not gonna pass.