How to file casualty loss with house lost in a landslide?

Casualty and Theft losses are not deductible on a federal tax return unless the loss occurred in a federally declared disaster area.

You need to determine if part of a  federal disaster and /or you may have insurance reimbursement. 

You want to begin with Topic No. 515 Casualty, Disaster, and Theft Losses | Internal Revenue Service. Declaring a federal disaster loss is covered in the article and you would use Form 4684 for casualty loss.

However, you may have had a gain due to insurance, which would create taxable income. This is also covered in Topic 515. I am sorry this happened to you.