As soon as you put their emails into AADSAS, they automatically get sent an email with instructions on how to upload electronically. So I would just let your professors know to look out for that email. They can upload a word doc or pdf for the letter, and then when they submit it you get sent an automatic email from AADSAS letting you know that it's done! Pretty simple process, you just need to remind your professors (politely) to get it done 🙂