Click on Accounting, then Vendors, then select the appropriate Vendor from the list on the left.

Click the New Transaction button near the bottom right of the page, below any current bills.
This will open up a new bill. From here, you can enter the Bill No., the Item for the charge, the Site, a Description, the company associated, and the amount and dates. 

If you do not see an item type listed that you would like, you can scroll all the way down on the Item dropdown and click Manage..  Then scroll all the way down on the Manage Items lightbox and click New to create a new Item linked to a specific account that you can use for the charge. (You will need to refresh the page to use this item on the current bill).

If you want to add multiple items on the bill (for example A/C repairs and plumbing repairs), then click the three dot menu on the right side of the bill to add another line item, and repeat editing the item, company, site and amount for each item.

Then when you are done, click Save on the bottom right to save the bill.