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Soldering a keyboard is slightly different to soldering copper pipes

The good soldering threads have already been linked above.
Something you can do is find an old, broken electronic device, a DVD player or flatscreen TV. Pull it apart to get the PCB out, and then practice desoldering and soldering the components on it.
If you break something - doesn't matter as it was broken before you started.
Rather than trying your first precision soldering on an expensive keyboard kit!