Magic Set Editor itself is fairly intutive - the “Add card” button (in the Cards menu) creates a blank card, and you can click on the relevant sections of the card to fill in information.
When writing symbols, use the codes listed below. Much of the time MSE will detect it automatically (you may need to finish your line), but if it doesn’t you can highlight it and press CTRL + M. You can also use this to turn a symbol back into a letter. Alternatively surround it in
|Generic||, ,||1, 2, X|
You can change the style of the card (for split cards, timeshifted cards, planeswalkers, etc.) by going to the Style tab. The program comes with post-8th Edition templates, but you can download the post-M15 templates (and many others) at the MSE Forums. Check the MSE FAQ for more information.