Game meshes are stored in the Drawable (.ydr) file format. Creating Drawables in Sollumz is very simple. This tutorial will walk you through step-by-step.
The first thing to do is convert your object(s) to a Drawable. Select all the objects and click Convert to Drawable
in Sollumz Tools > Drawables > Create Drawable Objects
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You will notice in your Scene Collection top right, that your object has changed a little. It is now in the correct hierarchy / format for GTA V.
If you navigate to Mesh Properties > Sollumz LODs
with the drawable model selected, you'll also notice that the active mesh is automatically assigned to the "High" LOD level. This panel is where you can assign other meshes as LODs. More info on that here. For the sake of this tutorial, we will only be setting the High LOD level.