Toki pona's grammar is a lot like a programming language's grammar. It is incredibly rigid, and for that, all sentence can be described in a fairly short list of rules, except unlike Esperanto, which has a short list of rules, tp's rules really all there is. Real languages grammar is hard and describe by endless trees where branches can be detached and re-attached elsewhere, where trees can be dozens of levels deep. Tp on the other hand, is either tree-less or has very shallow trees.
Anyhow, here's a shot at a sort of decision tree description of the grammar.
Do you have a precondition? Add a sentence plus la. (See below for how to build a sentence)
When did it happen? add a tenpo X la phrase.
You must have a subject, which is a noun phrase.
Do you have modifiers? Are any of these modifiers like nouns or need modifiers of their own? if so, add pi phrases, which are noun phrases.
Is your subject described by a prep phrase? Uh, oh. Either resort to pi + prep phrase, or move that prep phrase to the end of the upcoming li phrase.
You must have a verb (or see the predicate section).
How do you feel about what is happening? Prefix the verb with modals (ken, wile usually).
How did it happen? Suffix the verb with adverbs (lili, suli, ike, etc)
Do you have a direct object? Add e phrases, which are noun phrases (see above) Repeat as needed.
Do you want to place the action in space (use a lon phrase)
Do you want to indicate who is also doing this or who is nearby? (use poka phrase)
Is this similar to something else? (Use sama phrase)
Was this accomplished with a tool of a physical or metaphorical sort? (Use a kepeken phrase)
Was this on behalf of, for the goal of, or in the direction of something? (Use a tawa phrase)
[I'm forgetting a prep here...]
Did the subject do anything else? Add more li phrases.
And if this is a predicate, the li phrase is just a noun phrase.