Every piece of writing needs to have an underlying framework whereby each section leads into the next in a logical and coherent way and each point follows logically from the one before it. It doesn't matter whether you are writing formally or informally: you still need to make sure that your reader can readily understand what you are trying to say.

Paragraphs
Most relatively long pieces of writing are divided into paragraphs to help guide the reader through the content and enable them to grasp the main points. A new paragraph signals the introduction of a new idea, theme, or argument and each new paragraph should be indicated by starting a new line: this applies to printed or electronic text and to handwriting. It's usual to leave a line between each paragraph to make the division clear.
Well-structured paragraphs have three sections:
    • the topic (or lead) sentence tells you what the paragraph is going to be about. It's usually the first sentence in a paragraph:
There are many reasons why people join a gym or health club.
    • the next part of the paragraph should support or develop the idea or ideas presented in the topic sentence:
There are many reasons why people join a gym or health club. They may want to lose weight, maintain their current level of fitness, improve their muscle tone, or just meet other people with similar interests. They may want to take advantage of other facilities offered by the gym, for example complementary therapies or beauty treatments.
    • the final sentences rounds off or restates the main idea of the paragraph and it should also act as a link to the next one:
There are many reasons why people join a gym or health club. They may want to lose weight, maintain their current level of fitness, improve their muscle tone, or just meet other people with similar interests. They may want to take advantage of other facilities offered by the gym, for example complementary therapies or beauty treatments. People view their gym membership as an important aspect of their lives, both in terms of health benefits and leisure activities.
Separate paragraphs in a piece of writing need to be linked so that each one flows logically into the next. For example:
By repeating the noun phrase ‘gym membership’, the writer links the second paragraph to the content of the previous one and makes it easier for the reader to follow the argument they are beginning to develop.

On top of the underlying structure of a piece of writing, we also use various devices to link individual sentences and parts of sentences. This makes our writing flow naturally, without unnecessary repetition, and it also helps the reader to grasp the relationship between the ideas we're expressing. Here are the main ways in which we do this:
Reference
When we're writing or speaking, we use certain types of word to refer back to things that we've already mentioned or explained: this is something that we all do automatically. Pronouns have a particularly important role to play here. Take a look at this paragraph:
If pronouns hadn't been used, the paragraph would read:
Hyphens are used to link words and parts of words. Hyphens are not as common today as hyphens used to be but there are three main cases where you should use hyphens.
Four uses of the noun ‘hyphen’ makes the paragraph repetitious and awkward: replacing the noun with pronouns results in a tighter and more elegant structure.
In general, using pronouns in place of nouns that have already been mentioned makes a piece of writing shorter and more focused. But remember to make sure your reader can follow any pronouns without losing track of who or what you're referring to: too many uses of ‘he’, ‘she’, ‘it’, ‘them’, etc. can become confusing.
Linking
There are many expressions we can use to link different parts of a continuous piece of writing together. These expressions work in various ways, the most important of which are explained here.

Adding or listing different points
When we are giving an explanation, for example, or developing an argument in a piece of writing, we often need to organize our points in a particular order. Using words like firstly and secondly helps indicate the relative importance of the different points, and make the piece of writing into a coherent whole. For example:
Sometimes it's not necessary to specify the relative importance of the different points we are making, but we may nevertheless want to show that they are linked. For example:
Other words or phrases used in this way include:
as well
at the same time
additionally
besides
finally
first
furthermore
in addition
last
meanwhile
moreover
next
soon
then
too
what is more
Giving examples to illustrate a point
When you are making a particular point in a piece of writing, you often need to introduce a fact or detail that illustrates your argument, e.g.:
Other expressions which can be used in the same way include:
as follows
by way of illustration
namely
such as
to give an example

Explaining the cause and/or result of something
One sentence in a paragraph is often the logical development of the previous one and there are various terms we can use to highlight this connection. For example:
Other terms and expressions that can be used in this way include:
accordingly
as a consequence
consequently
for this reason
hence
in consequence
so
that being the case
therefore
thus

Drawing contrasts
You can use linking words or expressions to contrast one sentence with a previous statement, for example:
Other words and expressions that can be used in this way include:
all the same
alternatively
anyway
by contrast
conversely
despite this
even so
however
instead
in spite of this
nevertheless
nonetheless
rather
yet
Some of the terms in this list can also be used to concede that a fact or statement is true or justifiable, rather than to draw a contrast. For example:
When you are building a piece of writing, think about the relation between one idea in a sentence and the sentences that follow, and between one paragraph and the next. Then choose the appropriate linking words. This will enable your reader to move easily from one sentence or paragraph to the next, and it will enable you to get your message across in a way that is coherent, clear, and effective.