How do I pour concrete in an existing garage?
Sometimes hiring pros is the best option.

There are various approaches to the problem. You can set screed guides/rails that you first use to get it flat, then pull and fill in the void they leave. Some are designed to be left in place as expansion joints.

You can use a dense foam insulation around the edge of the slab and use that to guide your screeding.

You can attach 2x4 to the walls above your desired surface and use a screed with an offset guide that rides on the raised 2x4.