The best thing you can do is to follow the github tutorials which are most likely to be up-to-date.
First you should follow the TLDR part.
Note that the frontend developpers need to know about the usage specification.
Finally you want to go through the documentation. Here are some samples that might help:
Routes
Rails.application.routes.draw do
  # Stuff
  devise_for :admin_users, ActiveAdmin::Devise.config
  ActiveAdmin.routes(self)
  devise_for :users
  root to: "home#index"
  # The API part
  namespace :api, defaults: {format: :json} do
    scope :v1 do
      mount_devise_token_auth_for 'User', at: 'auth', skip: [:omniauth_callbacks]
      resources :stuff, only: [:index, :show]
    end
  end
end
A controller: 
module Api
  class StuffsController < ApiController
    before_action :authenticate_user!
    ...
  end
end
API Controller
class ApiController < ApplicationController
  include DeviseTokenAuth::Concerns::SetUserByToken
end
User model
class User < ActiveRecord::Base
  # Include default devise modules.
  devise :database_authenticatable, :registerable,
          :recoverable, :rememberable, :trackable, :validatable
  include DeviseTokenAuth::Concerns::User
end
Finally don't forget to configure the gem in the corresponding initializer.