The Global Historical Climatology Network (GHCN) data is described at http://www.ncdc.noaa.gov/temp-and-precip/ghcn-gridded-products.php . To obtain the precipitation station data, please write to one of the contacts on that page.
The following is my interpretation of what is described on the gridded GHCN WWW page. No adjustments have been made to the data in the Sahel region to take into account movements of stations and changes in measurement practices. Only stations with at least 20 years of data in 1961-90 are used. Anomalies in units of cm/month are calculated for each calendar month for these stations, and the stations within each 5° latitude-longitude box are averaged.
I average the anomalies for the Sahel region for each month and year, and take June through October averages for each year.
The GHCN gridded data set is lightly processed. Analyses of Sahel precipitation anomalies in more processed data sets, for example where optimal interpolation is employed, are very similar to the GHCN product.