Do you have a recommendation on a map to show us how to walk from mission to mission?
The best map on the missions is California Missions, which contains original artwork created by Francois Davot. It sells for $5.95.
While this (and most of the map products) have some directions I have never found anything that is really as good as doing an online search once you have worked out the direction (north or south), the sequence of missions to be visited (which will be affected by other stops along the way, hotel plans etc.) and the amount of time you want to take. With the map in hand get the specific address of each mission (this is available on our website from the front page tab Mission Gateway / Mission Information.) then obtain very specific directions for your itinerary through Mapquest or some other comparable service, which are all free.
I assume you were not being literal when you said walk. The missions extend over about 600 miles and were sited to be about a day apart (by horse / cart). I have visited all twenty-one three times and it takes about six days to do right, five days if you take some shortcuts and 'rush'.
There are some missions situated in areas where there is an awful lot of mission era things to see: Mission San Diego, Mission Santa Barbara, San Juan Bautist and Mission Carmel / Monterey. The only fully reconstructed mission complex is La Purisima (now a state park).