The first to thing to have: a model that contains a has_many relation with another model.

class Project < ApplicationRecord
  has_many :todos
end

class Todo < ApplicationRecord
  belongs_to :project
end

In ProjectsController:

class ProjectsController < ApplicationController
  def new
    @project = Project.new
  end
end

In a nested form, you can create child objects with a parent object at the same time.

<%= nested_form_for @project do |f| %>
  <%= f.label :name %>
  <%= f.text_field :name %>

  <% # Now comes the part for `Todo` object %>
  <%= f.fields_for :todo do |todo_field| %>
    <%= todo_field.label :name %>
    <%= todo_field.text_field :name %>
  <% end %>
<% end %>

As we initialized @project with Project.new to have something for creating a new Project object, same way for creating a Todo object, we have to have something like this, and there are multiple ways to do so:

For strong params, you can include them in the following way:

def project_params
  params.require(:project).permit(:name, todo_attributes: [:name])
end

Since, the Todo objects will be created through the creation of a Project object, so you have to specify this thing in Project model by adding the following line:

accepts_nested_attributes_for :todos