What do you mean with "create an own custom attribute"?
Normally you may use the query builder orderBy() method with Eloquent models too, or at least the Collection sortBy().

I mean that if I for example create something like this in my model:

public function getCountAttribute()
{
    return $this->someNumber + 
        OtherModel::where('something', $this->id)->pluck('someNumber');
}

I can then reach that value by $model->count, but can I somehow order by that attribute?

Your example leads to sorting the collection at application level, but that may perform poorly without eager loading.
So, first try to define a proper Eloquent relation and simplify your accessor, e.g.

class SomeModel extends Eloquent {
    
    ...
    
    public function getCountAttribute()
    {
        return $this->someNumber + $this->relatedModel->someNumber;
    }
    
    ...
    
}

At that point, you could try something like this:

$orderedModels = SomeModel::with('relatedModel')->get()->sortBy('count');

This may need some tuning but I hope it points you in a good direction.

Yeah, I will not use what I had in my example, it was just a quickly put together example to show what I wanted to achieve.

I was not aware of the sortBy method, that's exactly what I am looking for. Thank you very much.