The literary device called analogy frequently refers to an extended metaphor or simile:
While simile and analogy often overlap, the simile is generally a more artistic likening, done briefly for effect and emphasis, while analogy serves the more practical end of explaining a thought process or a line of reasoning or the abstract in terms of the concrete, and may therefore be more extended.
(From A Handbook of Rhetorical Devices.)
And this from "Literary Terms & Devices":
Analogy extends a metaphor.
Two quotes, from early and later thinking may help you understand how the meanings of 'simile', 'metaphor' and 'analogy' intertwine:
"A simile is also a metaphor; for there is little difference: when the poet says, 'He rushed as a lion,' it is a simile, but 'The lion rushed' [with lion referring to a man] would be a metaphor; since both are brave, he used a metaphor [i.e., a simile] and spoke of Achilles as a lion. The simile is useful also in speech, but only occasionally, for it is poetic. [Similes] should be brought in like metaphors; for they are metaphors, differing in the form of expression."
(Aristotle, Rhetoric, Book Three, Chapter 4. Translated by George A. Kennedy, Aristotle, On Rhetoric: A Theory of Civic Discourse. Oxford University Press, 1991)
"Most theorists have thought that metaphor is somehow a matter of bringing out similarities between things or states of affairs. Donald Davidson [above] argues that this 'bringing out' is purely causal, and in no way linguistic; hearing the metaphor just somehow has the effect of making us see a similarity. The Naive Simile Theory goes to the opposite extreme, having it that metaphors simply abbreviate explicit literal comparisons. Both views are easily seen to be inadequate. According to the Figurative Simile Theory, on the other hand, metaphors are short for similes themselves taken figuratively. This view avoids the three most obvious objections to the Naive Simile Theory, but not all the tough ones."
(William G. Lycan, Philosophy of Language: A Contemporary Introduction, 2nd ed. Routledge, 2008)
(Quote reproduced from Glossary of Grammatical and Rhetorical Terms; see especially the section below the heading "Observations on the Differences Between Similes and Metaphors".)
If you put all the foregoing together, you find that the answer to the first question you pose, "how can one turn any analogy into a metaphor?", is that any analogy is already a metaphor, or a simile, depending on how the analogy is expressed. As Aristotle points out, similes are identical to metaphors, differing only in the form of expression. This answer begs your further questions:
Is there anything to bear in mind about turning an analogy into a metaphor, bar grammatical code like making sure it's a complete sentence?
Yes: bear in mind the form of the expression. Use one form, explicit comparison, and your analogy will be a simile. Use another form, implicit comparison, and your analogy will be a metaphor. That's really all there is to it, despite the confusing complexities and mysteries I will later introduce. This answer (to your second question), also bears directly on your third question,
Is there any surefire way to turn an analogy into a metaphor proper? E.g.: a colon, to signal that I am explaining the tenor; or simply writing a simile then omitting the explicit comparison.
As explained, an analogy is already a metaphor, or a simile, depending on the form of the expression. So, your idea that inserting a colon in place of an explicit comparative phrase (for example) might sometimes work, but won't always. Unless you understand what you are doing, and saying, the result of inserting that colon will sometimes be merely a mess. Far better will be to understand the different ways of expressing explicit and implicit comparisons, and of converting one form of expression to another.
Theorists have argued that language, learning, and thinking are essentially metaphorical processes. For a technical analysis of some current thinking on metaphorical processes in language and thought, see Metaphor in language and thought: How do we map the field?, by Gerard Steen (in press).
See "The role of metaphor in learning and knowledge formation" for a summary description of one project focussed on analyzing how metaphorical thinking underlies learning processes. For more detail, see "Cathedrals in the Mind: The Architecture of Metaphor in Understanding Learning" (Chapter from Cybernetics and Systems '86, pp. 285-292, by Kathleen Forsythe). The paper that became the chapter received the American Cybernetic Society award for the best paper of the year (1986). This is the complete abstract:
The pervasiveness of metaphor in our conceptual system suggests a central and basic role in the underlying architecture of thought. Metaphor represents the ability to understand one thing in terms of another as we ascribe an understood pattern to an unknown phenomena and perceive their structural integrity within the environment of our experience. We can then begin to perceive the environment of learning as one in which analogical thinking serves as architecture, analytical thinking serves as engineering and the imagination ensures that the interactions which create life and meaning are always being realized anew. The implications for this approach to applied epistemology provides insight into the design and development of learning systems that support the creative nature of learning.