Shuttering around garage door before floor

[Will1986]

Morning guys,
in the process of building an extension and a bit confused when it comes to the garage floor.
There is not foundation in the gap where the door will be so what do I do about shuttering it so I can fill in the space and then pour concrete.
Also if I put a dpm down how do I finish it around the door?

Thanks

 

[John D v2.0]

Plywood should do it.
And with the dpm I guess there will be a very small cill/step required just inside the door. If the door manufacturer doesn't give that detail, then I'd personally finish the dpm at the high end of the step or thereabouts. If it was a house more thought would be needed, but a garage just needs to be dry enough to not be damaged