To setup onepage menu, you first need to create and setup menu. To do this, read article here.
After that to setup onepage menu
Step 1: Go to admin panel > menus > Checked on “Enable Onepage For This Item (only for main parent)”.
Tip: Add only target row id with # instead of whole url.
Step 2: Go to page in which you want to link this one page navigation.
For that click on “Extras” tab in Row Edit popup as shown in below image, add menu item URL without # tag (suppose you have used #home as menu url then you need to use home as extra id). Repeat the same step for all rows of page to assign ID to row.