I use calibre to convert to mobi if it is not already in that form. Then I just email it to that device. Works for me.
If you don't know the email address, just look under settings in your kindle app. Your email address will be there.
For example, if my regular email is
myemail123@yahoo.com, my email address on say my phone might be
myemail123_45@kindle.com and my tablet might be
myemail123_26@kindle.com.
After you email, next time you bring up your app it will be there. If it appears that the cover didn't get through, click on the book and open it, then close it. The cover ususally appears then.
There are various forms that the Kindle apps will accept as well, not just mobi. Hope this helps.