How to store the relation between tables

I created a table in relation to two other tables, however I do not know how to relate them when creating new projects, how could I do that?

I'll create the categories in a separated page in my admin, and when the user create's a new project he will be able to select an array of categories coming from the table.

My question is, how can I store the relation when POST the data? I've never done this before.

Project model

class Project extends Model
{
    protected $table = 'projects';

    protected $fillable = [
        'name',
        'slug',
        'header',
        'desc',
        'about',
        'url',
        'status'
    ];

    public function customer()
    {
        return $this->belongsTo(Customer::class);
    }

    public function category()
    {
        return $this->belongsToMany(Category::class);
    }

    public function categories()
    {
        return $this->hasMany(Category::class);
    }
}

Category model

class Category extends Model
{
    protected $table = 'categories';

    protected $fillable = [
        'name',
        'status'
    ];

    public function subCategory()
    {
        return $this->hasMany(SubCategory::class);
    }

    public function projects()
    {
        return $this->belongsToMany(Project::class);
    }
}

My actual Post create

public function postCreate(ProjectCreateRequest $request, Customer $customer)
{

    //Array
    $categories = $request->categories;

    $customer->projects()->create([
        'name' => $request->name,
        'header' => $request->header,
        'desc' => $request->desc,
        'about' => $request->about,
        'url' => $request->url,
    ]);

    //How do I store the relation?

    return redirect('admin/clientes/editar/' . $customer->id);
}

via Chebli Mohamed