May 27, 2020
Good question. Yes, in most cases, it's appropriate to give your teachers stamped, addressed envelopes and ask them to send their recommendations directly to colleges. (Some high schools, however, prefer to have all references filter through their guidance office. Then teachers simply give their completed forms and/or letters to the guidance staff. Presumably, this is not the case in your school.)
Thus, once your teachers launch their letters in the mail, the colleges receiving them will know what to do. Of course, it's essential that your name and school name are clearly indicated on each reference. It would also be helpful for you to write "Recommendation for [and your name and school name] on the front of each envelope, near where you've put the college address. That way, admission secretaries will know exactly where to route the missive when it arrives.
Admission offices are very accustomed to receiving application components that show up on their own. Only in rare cases (I can think of just one school in CA that requires this) do colleges request that everything be sent together. Moreover, given the allegedly confidential nature of teacher recs, it's actually expected that they will be mailed separately.
In a typical admission office, once your application fee is received, the college starts a file or "folder" with your name on it. From that point on, every scrap of paper associated with you--from reference forms to phone messages--goes into that folder. However, sometimes material arrives even before the fee is paid and the folder is opened. In such cases, the submissions go into an alphabetized "general file." Once an application fee arrives, the admission staffers plow through that general file to see if the candidate has any materials waiting there.
Considering how many documents are part of each admission cycle, admission offices do an extraordinary job of keeping tabs on what goes where. However, you can help by labeling every morsel you submit with your name and school name. It is also incumbent on you to make follow-up phone calls about a week to 10 days after your applications are mailed (or submitted electronically) to make sure they arrived safely. Even if that week to 10 days takes you past deadline, no problem. Admission folks allow wiggle room when documents go missing. Some colleges notify you when your application is complete or offer online tracking with a PIN number. However, the responsibility still rests with YOU to ascertain that everything is in order.