Currently, the only app that can emulate SmileBASIC accurately is Citra, the 3DS emulator. I'm not going to give you the ROM location, but it's pretty easy to find one. back up your 3DS copy so you can emulate it. Anyway, all "emulation is(n't) legal" boilerplate aside, here's 3 things that will help with your Citra-using experience.

One - Getting the window to that perfect size

Hate black bars around your viewport? Do I have the tool for you. Here's Positive Aspects, a tool that can resize any window according to any supplied aspect ratio. I recommend adding 400x480 to the resolutions file, and I usually set it to 4x scale.

Two - Typing on this horrible keyboard

Hate having to hunt for the key you want to press, then moving your mouse and clicking it? Hate repeating that 100 times until your code is written? Do I have the tool for you? No

old

Probably not, because this is a taped-together disaster of code. You'll need AutoHotkey for this to work. Here's a link to my script! Just modify the scale variable that I tried to make automatic, and enable caps lock! Now open your SmileBASIC window and start typing! Don't type too fast, it can't keep up sometimes. If it forgets what shift state you're in, just toggle the caps lock on the virtual keyboard.

Yttria made a better one

Three - Importing and Exporting files

First: Navigate to your SmileBASIC user folder. It'll be somewhere around here.

C:/Users/???/AppData/Roaming/Citra/sdmc/Nintendo 3DS/ just a ton of zeros / more zeros /extdata/00000000/000016DE/user

Second: Find your file. Want to import a .txt or .png? Find the directory that contains that file. Want to export a PRG: or GRP:? Find that in your user directory, which should be familiar. The three hashtags are your [DEFAULT] folder. Third: Open this web site. Fourth: Are you using a SmileBASIC file? Drag your file into the "Edit" field. Otherwise, select the file you want to make. Fifth: Download the resulting thing at the bottom of the page, unless you're editing a DAT file. If so, you probably know what to do with the resulting data.