Go to PubMed: https://www.ncbi.nlm.nih.gov/pubmed/ and sign in to NCBI in the upper right corner of the page (if you have not signed in before, you can create a MyNCBI account for yourself.
My NCBI is a very useful tool in PubMed and will allow you to save and update searches on a regular basis. It will also keep track of the searches you do when you are signed in.
Once you have established your account, go to your MyNCBI Dashboard and look for the window labeled "Filters". (If you do not see a window for "Filters", click on "Customize this page" in the upper right, and there you will have an option to "turn on" the "Filters" window.)
Within the "Filters" window, click on the "Manage Filters" link below the "Filters" box. The Manage Filters page will open with Popular Filters chosen. You may want to select a few of those e.g. Free Full Text. (This does not limit your search to only Free Full Text - it
shows up as a link in the upper right corner of your results screen.)
Once you have chosen your Popular Filters click to fill the radio button for Link Out,
then type Harvard in the search box that appears below.
Click in both of the square boxes to the left of Harvard Libraries. This will cause the
Findit@Harvard icon to appear in the abstract pages within PubMed.
You will be asked to log in the first time you access an article from your search but
the login credential should last through at least 30 minutes of inactivity and
will last indefinitely if you are actively using the link.