If you want to install a WordPress theme from the WordPress.org theme directory, then using the WordPress admin theme search is the best functionality to go by. Here is how you can do that

1. Log into WordPress Admin area.
2. Go to appearance and click themes.
3. On the theme page, click Add New button, it’s located at the top.
4. This directs you to the category of themes screen. Select from the different categories of themes.
5. Create a search criteria to determine the best theme to use.
6. Hover the mouse on the image. Select the install options from install, preview, and details options.
7. Click install button or install option.
8. WordPress will install the theme, activate the theme to use it.

To install commercial or premium themes, you will have to use the Upload method from the WordPress Admin. Here is how you can go about this.

1. Download the .zip file of the theme that you have bought.
2. Login to your WordPress Admin area
3. Click on appearance and then themes.
4. On the theme page, click the Add New Button.
5. Click upload Theme button on the next screen.
6. Choose the zip file that you had earlier on downloaded. Select the file and then click install.
7. Once the theme has been installed, activate it.