METHOD 1 #
We can reach the widget page by clicking:
Step 1 — Navigate to Appearance > Widgets in your WordPress dashboard.
Step 2 — The widget page may look different for you, so if you install and activate the classic widget plugin from https://wordpress.org/plugins/classic-widgets/ , the page will look same for you.
Step 3 — To add new widgets in a sidebar, you can drag a widget to a sidebar that you want to be shown.
METHOD 2 #
Step 1 — Navigate to Appearance → Customize from WordPress Admin Dashboard.
Step 2 — Click the Widget → Sidebar tab from the leftside menu.
Step 3 — From this section you can add new widgets to the related sidebar by clicking Add a Widget button.
Step 4 — After all the changes that you made, you can save the changes by clicking the Publish button.