How to Implement pagination Plugin in custom theme

I had install pagination plugin & also search plugin but I don't know how to implement in my custom theme

Could you please help me to integrate plugin.

I would start by reading the Readme of each plugin if you haven't already. Installed correctly, pagination should work pretty easily by adding 'pagination: true' to your collection in the frontmatter.

---
title: My Gravtastic Blog
content:
  items: '@self.children'
  order:
    by: header.date
    dir: desc
  pagination: true  // <---- this 
  limit: 10  // <---- and this
---

Simplesearch can be a little more complicated depending on what you are trying to accomplish but it usually about as simple.

Since you have a custom theme you'll need this in your template.

{% if show_pagination and config.plugins.pagination.enabled and collection.params.pagination %}
   <div id="listing-footer">
      {% include 'partials/pagination.html.twig' with {base_url: page.url, pagination: collection.params.pagination} %}
   </div>
{% endif %}