Intall it just as regular extension:

1. Upload .ocmod.zip file via extension installer

2. Refresh Modification

3. Install HP Sitemap Generator Pro, in extensions -> extensions -> feeds

4. Enable HP Sitemap Generator Pro.

5. Then modify .htacesss

Replace

RewriteRule ^sitemap.xml$ index.php?route=extension/feed/google_sitemap [L]
With
RewriteRule ^sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro [L]
RewriteRule ^product-sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product [L]
RewriteRule ^product-sitemap-1.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product&page=1 [L]
RewriteRule ^product-sitemap-2.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product&page=2 [L]
RewriteRule ^product-sitemap-3.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product&page=3 [L]
RewriteRule ^product-sitemap-4.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product&page=4 [L]
RewriteRule ^product-sitemap-5.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/product&page=5 [L]
RewriteRule ^category-sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/category [L]
RewriteRule ^information-sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/information [L]
RewriteRule ^blog-sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/blog [L]
RewriteRule ^faq-sitemap.xml$ index.php?route=extension/feed/hp_sitemap_generator_pro/faq [L]

 note : Add rewrite rule: product-sitemap-{n}.xml as many as possible products page.