How to add multiple categories to a model using a form in Django?

I have a model called BlogPost and one called Category, each BlogPost can have multiple categories. When creating posts from the admin panel I have no problems, but when trying to implement a form that creates a BlogPost so users can create posts I get an error:

Direct assignment to the forward side of a many-to-many set is prohibited. Use categories.set() instead.

I tried reading the documentation but there's something that I'm not understanding because it's not working.

My models are this;

class BlogPost(models.Model):
    title = models.CharField(max_length=250)
    content = models.TextField()
    post_author = models.CharField(max_length=30)
    creation_date = models.DateTimeField(auto_now_add=True)
    modification_date = models.DateTimeField(auto_now=True)
    image = models.ImageField(upload_to='blog/files/blog_images')
    categories = models.ManyToManyField('Category', related_name='posts') #Relaciona el atributo con la clase Category y permite usar category.posts

    def __str__(self):
        return self.title
    
    class Meta:
        verbose_name = "blog post"
        verbose_name_plural = "blog posts"

class Category(models.Model):
    cat_name = models.CharField(max_length=50)

    def __str__(self):
        return self.cat_name
    
    class Meta:
        verbose_name = "category"
        verbose_name_plural = "categories"

This is the form:

class newPostForm(forms.Form):

    CATEGORIES = (
        (1, "Agua dulce"),
        (2, "Agua marina"),
        (3, "Primer acuario"),
        (4, "Alimentación")
    )

    título = forms.CharField(max_length=250)
    contenido = forms.CharField(
        widget=forms.Textarea()
    )
    imagen = forms.ImageField()
    autor = forms.CharField()
    categorias = forms.ModelMultipleChoiceField(
        queryset=Category.objects.all(),
        widget=forms.CheckboxSelectMultiple
    )

And this is the view:

def new_post(request):
    if request.method == "POST":
        form = newPostForm(request.POST, request.FILES)
        if form.is_valid():
            
            info = form.cleaned_data

            new_post_info = BlogPost(
                title=info["título"],
                content=info["contenido"],
                image=info["imagen"],
                post_author=info["autor"],
                categories=info["categorias"]
                )

            new_post_info.save()   

            return redirect(request.path_info)
        
    else:
        form = newPostForm()
    
    return render(request, "blog/new_post.html", {"form":form})

Any suggestion is welcomed!

I tried with the saving before and adding the categories as described in the documentation but it didn't work.

I solved it, this is how for anyone wondering:

Modified the view to:

def new_post(request):
if request.method == "POST":
    form = newPostForm(request.POST, request.FILES)
    if form.is_valid():
        
        info = form.cleaned_data

        new_post_info = BlogPost(
            title=info["título"],
            content=info["contenido"],
            image=info["imagen"],
            post_author=info["autor"],
            )

        new_post_info.save()  
        new_post_info.categories.set(info["categorias"])

        return redirect(request.path_info)
    
else:
    form = newPostForm()

return render(request, "blog/new_post.html", {"form":form})

It now takes what is stored into the "categorias" field form the form and passes it to the BlogPost object attribute categories after saving the new_post_info object.

PD: I don't know if it's customary for one to answer their own question, but this may help someone in the future.

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