I'm an amateur baker who's never really tried baking cakes before, although I have intermediate decorating skills and can make a beautiful cupcake. I want to make a sheet cake for a friend's birthday, which is upcoming soon. I've looked up all the literature (read: YouTube videos) on the subject and it appears that to get the most professional-looking frosting layer with buttercream, you should use a cake turntable. I don't have a cake turntable, however, and may not be able to get one until it's too late for the party. Is there an alternate method I could use to get a good-looking layer of frosting without a turntable? (The one resource I've seen that didn't use one simply rotated the cake board around - is this viable?)

Another question I have is, and this may seem naive of me to ask, but how is one meant to transfer a sheet cake into a box? I'm planning to make a two-layered quarter sheet cake (9" x 13"). Is picking up the cake board and placing it in the box sufficient, or can I make it even easier and buy some sort of flattened box to assemble around the cake, if such a thing exists?

 Some people just buy a lazy susan which works well and acts like a turntable type of thing. I bought mine at Big lots and only paid ten dollars for it. It is made of glass but some are made of wood.

You should be fine just picking the cake board up with decorated cake on it  and putting it in the cake box.  Some people put two cardboard cake plates together which they glue to make it stronger if you want to.   Maybe others here who are more experienced can tell you what they do. I am just a hobby baker.

The turntable thing is really for speed. You will be fine without it, especially for a rectangle cake. The turntable really helps with a round cake, but again, not necessary.

Hobby Lobby sells boxes...one at a time. Usually, you assemble 3 sides and slide the cake board in and finish the fourth side and fold over the top. 9 by 13 is not that hard to handle. As remnant said, a double board, glued together helps. Either two actual cake boards, one priority US mailing box cut down and covered, or even foam core boards. It  looks really nice covered with a decorative ribbon wrapped around the outside edge and glued in place (hot glue is my choice). AND you can prepared that ahead of time.

Good Luck!

Since you're doing a 9x13, a standard turntable won't help much.  It's a nice to have but not really a need to have in your circumstance!  But what I would do is turn over a sheet pan, and decorate the cake on that.  If you're decorating to the very edges of your cake (ie, not leaving space for a piped border or similar) you can just frost onto the sheet pan and then just wash that off, leaving a nice clean line on your cake. :-)  And, you can just manually turn the sheet pan around to get to the sides.