What should I do before using this plugin?

You definitely should make a backup of your database before running this plugin for the first time. If nothing bad happens, you can run this plugin after that as much as you want without any need for backup of your database.

I cannot get rid of transient options from my database!

Your WordPress site always make some transient options, and as soon as you clear them from your database, it will make some new ones. There is no way to prevent this. However, after initial cleaning and optimization, the number of these options should be lower. That’s why we provided you with Auto Clean-up option.

My database size information is shown as different as it really is!

That’s usually because of the corrupted tables in your database. This plugin cannot do anything about that. However, your hosting company should be able to repair your database.

Is InnoDB format supported?

No. InnoDB format is not supported. Database optimization will not work for these tables, but everything else should work fine.

KleenPress is not working for me! It doesn’t optimize anything!

The reason for this is not in the plugin. It’s your hosting company (especially if your website is on shared hosting). The problem is (most likely) that your hosting company prohibits scripts to run OPTIMIZE commands in SQL. Talk to your hosting company about that. They should enable that for you.