I usually suggest window envelopes so that the address header on the enclosed letter shows through. This cuts out a whole process of printing the envelope and looks professional. (It must be good because lots and lots of people use window envelopes)
For folding the letter (assuming a trifold sheet) it's easy with experience to fold at the correct position but it is always possible to put a small marker on the letter template at an approriate place to indicate one third down (or up) the page. This is 99mm on A4.