How do I create a custom admin filter?

To create a custom admin filter you must create a class that extends \Aero\Admin\Filters\AdminFilter and implement a handle method:

class OrderStatusAdminFilter extends \Aero\Admin\Filters\AdminFilter
{
    use \Aero\Admin\Filters\Traits\HasDefault;
    use \Aero\Admin\Filters\Traits\AdminFilterHelpers;

    protected $view = 'admin::filters.order-status';

    protected function selectedOptions(): array
    {
        return request()->input($this->key(), []) ?? [];
    }

    public function handle(\Symfony\Component\HttpFoundation\ParameterBag $parameters, $query)
    {
        $view = $parameters->get('view');

        if ($view === null && strlen($parameters->get('q', ''))) {
            $view = 'all';

            request()->request->set('view', $view);
        } elseif ($view === null) {
            $view = static::getDefault();
        }

        $selectedStatusIds = $view === 'incomplete'
            ? collect()
            : collect($parameters->get($this->key()))
                ->map(function ($value) {
                    return intval($value);
                })
                ->filter()
                ->values();

        $query->when($view === 'incomplete', function ($query) {
            $query->whereNull('ordered_at');
        })->when($view === 'outstanding' || $view === 'new', function ($query) use ($selectedStatusIds) {
            $outstandingStatusIds = \Aero\Cart\Models\OrderStatus::forState(...setting('admin.outstanding_order_states'))->pluck('id');

            $statusIds = $selectedStatusIds->isNotEmpty()
                ? $outstandingStatusIds->intersect($selectedStatusIds)
                : $outstandingStatusIds;

            if ($statusIds->isNotEmpty()) {
                $query->whereIn('order_status_id', $statusIds);
            }
        })->when($view === 'all', function ($query) use ($selectedStatusIds) {
            $query->whereNotNull('ordered_at');

            if ($selectedStatusIds->isNotEmpty()) {
                $query->whereIn('order_status_id', $selectedStatusIds);
            }
        });
    }

    protected function viewData(): array
    {
        $selectedStatuses = collect(request()->input($this->key()));
        $selectedView = request()->input('view', static::getDefault());

        $orderStatuses = \Aero\Cart\Models\OrderStatus::ordered()->get()
            ->map(function ($status) use ($selectedStatuses) {
                return [
                    'id' => $status->id,
                    'name' => $status->name,
                    'state' => $status->state,
                    'url' => $this->getUrlFor($status->id),
                ];
            });

        return [
            'selectedStatuses' => $selectedStatuses,
            'statuses' => collect($orderStatuses)
                ->filter(function ($status) use ($selectedView) {
                    switch ($selectedView) {
                        case 'all': return $status['state'] !== 'incomplete';
                        case 'new':
                        case 'outstanding': return in_array($status['state'], setting('admin.outstanding_order_states')->toArray());
                    }

                    return false;
                })
                ->toArray(),
        ];
    }

    protected function baseName(): string
    {
        return 'Status';
    }

    public function stateFields(): array
    {
        return [$this->key(), 'view'];
    }

    protected static function defaultValue(): string
    {
        return 'outstanding';
    }
}

And then create a view for rendering the filter, in this case admin::filters.order-status:

<div class="border-b border-grey-light" data-filter-id="status">
    <button class="flex justify-between items-center w-full p-4" type="button" onClick="toggleContent(this)">
        <span class="text-sm font-medium">{{ $title }}</span>
        <span class="[ js-accordion-arrow ] w-4 h-4 flex items-center justify-center transform rotate-90">@include('admin::icons.caret-down')</span>
    </button>

    <div class="p-4 pt-2 list-reset [ js-accordion-content ] hidden">
        <div class="select w-full -mt-1">
            <select id="view_all" name="view" class="w-full" autocomplete="off" onchange="updateUrlQuery('view', this.value)">
                <option value="outstanding" {{ request('view', \Aero\Admin\Filters\Order\OrderStatusAdminFilter::getDefault()) === 'outstanding' || request('view', \Aero\Admin\Filters\Order\OrderStatusAdminFilter::getDefault()) === 'new' ? 'selected' : '' }} data-class="new">Outstanding Orders</option>
                <option value="all" data-class="all"
                        @if(request('view', \Aero\Admin\Filters\Order\OrderStatusAdminFilter::getDefault()) == 'all') selected data-filter-active="true" data-filter-group-name="View" data-filter-name="All Orders" data-filter-key="view" @endif>All Orders</option>
                <option value="incomplete" data-class="incomplete"
                        @if(request('view', \Aero\Admin\Filters\Order\OrderStatusAdminFilter::getDefault()) == 'incomplete') selected data-filter-active="true" data-filter-group-name="View" data-filter-name="Incomplete Orders" data-filter-key="view" @endif>Incomplete Orders</option>
            </select>
        </div>

        @if(count($statuses) > 0)
            <div class="pt-4 flex flex-col bg-white mt-2 overflow-hidden">
                <ul class="list-reset overflow-auto leading-loose" style="max-height: 216px">
                    @foreach($statuses as $status)
                        <li class="mb-2 filter">
                            <label for="status-{{ $status['id'] }}"><label class="checkbox">
                                    <input id="status-{{ $status['id'] }}" type="checkbox" name="{{ $key }}[]" autocomplete="off" value="{{ $status['id'] }}" onclick="location.href = '{{ $status['url'] ?? '' }}';"
                                           @if($selectedStatuses->contains($status['id'])) checked data-filter-active="true" data-filter-group-name="{{ $title }}" data-filter-name="{{ $status['name'] }}" data-filter-url="{{ $status['url'] }}" @endif
                                    > <span></span></label>
                                {{ $status['name'] }}
                            </label>
                        </li>
                    @endforeach
                </ul>
            </div>
        @endif
    </div>
</div>

Which receives the viewData defined by your filter type as well as the options defined by the base class ($title and $key).