Adding new blocks

Hello,

I'm using the t3-blank template.
New here.
I have read the documentation and saw the video about adding blocks, but I can't figure out how it works. Can't get it to work.

I want to use the lay-out home-1 but directly under the block mainmenu i want to add a block for a slider (see attachment).
I get an error message: Blok [slider1] not found.
I have added the position to the TemplateDetail.xml file.
In which .php blockfile I have to add code and how and where?

Can somebody please help me?

Greetings,

Hadewijch