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This is a sketch of the idea. Suppose are two complete ordered fields and we are trying to construct an embedding of in . Then for reasons to do with the ordered field structure, the copy of the rationals in and the copy of the rationals in are the same so we must map 's copy of onto 's copy of in the obvious and only way possible.
Now for there is a monotonic nondecreasing sequence of rationals converging to by one of the density theorems. Since we are forced to send to the copy of in (as ) we must therefore send to the limit of the copy of the sequence in . Fortunately (by the completeness axiom in ) this sequence does indeed have a limit so there is some copy of in to send to. This construction will give a canonical embedding of in , and if is also complete this embedding will turn out to be an isomorphism.