In order to apply for the Startup Grant (Starttiraha), you need the Business Plan (Liiketoimintasuunnitelma or LTS) document which is somewhat required but nobody asks you for it (except the Startup Grant officials and if you're applying for a company loan, for example) at least in the beginning of your entrepreneurship.
I will later post what kind of template I used and which was good enough to be accepted in order to receive the grant.
The first step towards the Startup Grant is to go to the nearest business advisor provided by the state. You find them in the bottom of this page: https://uusyrityskeskus.fi/in-english/. If you live in Helsinki like I do, you will most probably need to go to here: https://newcohelsinki.fi/en/services/starting-a-company-info-event/
Again, remember that if you want to have the grant, you aren't allowed to found your company or start your business (even publish the website or do any marketing) until you've been accepted for the grant.
I will continue this list later on, sorry about that