A few things of note here. One is that much of the reclaimed land of the Netherlands is not just sea floor, most of it is from bogs and fens or lakes that were surrounded with dykes and drained. This means that the soil in these areas is not universally salty. However, in areas that were reclaimed from the sea there is a process to render it more suitable to farming and other uses. Fortunately for the Dutch, the area receives enough rainfall that this process is mostly passive, just allowing sufficient time for natural rain to carry away excess salts.

An important category of reclaimed land is the "kwelder" or salt marsh. These are coastal areas where mud and silt is deposited by floods but have built up to a height above the average high tide, so while they flood regularly they do not flood every day. These areas often have vegetation growing on them already (grasses, for example), and because the surface soil is above sealevel the majority of the time the soils are less saline. Some of these areas can be converted into productive farmland within a matter of years (or even a single year) after being enclosed and drained, as long as the farmers are careful to make use of only a very shallow surface layer of soil. Though other soils can require considerably more effort.

The most important preparatory step here is cutting drainage channels in the new land to allow it to become less saturated and to provide a means for rain water to flow through. After that it's mostly just a matter of watching and waiting for a few years. The soil becomes cracked as it dries, setting the stage for being rehydrated by rain which flushes the salt out. For many soils it can take only a few years for rain to wash out the salt, for clay rich soils it can take longer (up to decades) for the sodium salts in the clay to be replaced by calcium salts (either from natural lime that occurs in the soil or from lime added as a fertilizer). Clay rich soils reclaimed from the sea might be allowed to remain as prairies for even up to several decades while the rain and natural processes are converting it to have less sodium and improving the sub-surface structure and composition.

Obviously there's a lot of complexity and subtlety involved (and great skill accumulated over centuries by the Dutch) that I'm skipping over, but for the most part the process is a matter of patience, carefulness, and paying attention, nature will do most of the heavy lifting for you if you let it.