Every install guide is written by somebody who has installed the thing fifty times. Every install guide is read by somebody doing it for the first time. That gap is where your support tickets come from.
Number every step, and make each one an action
"Configure the database" is not a step, it is a heading. "Open config.php and set DB_NAME, DB_USER and DB_PASS to your database details" is a step. If a line does not tell somebody to do something specific, it belongs in prose, not in the numbered list.
State the starting point
What must already exist before step one: a database created, a domain pointed, PHP at a minimum version. Buyers who do not have those will otherwise fail at step four and blame step four.
Say what they should see
After each significant step, one line describing the expected result. "You should now see the setup screen at yoursite.com/install." A buyer who sees something else knows immediately where it went wrong, rather than continuing and failing later.
Cover the two or three known failures
A short troubleshooting section listing the errors people actually hit, with what causes each. Your ticket history is the source for this and it is the highest-value part of the whole document.
Test it on somebody else
Hand it to a developer who has never seen your code and watch them, without helping. Every moment they hesitate is a missing line. This takes an hour and is worth more than a week of polishing the guide alone.
