Introduction to GEDmatch:

https://blog.genomelink.io/posts/how-to-use-gedmatch

Getting info from Ancestry:

http://www.irelanddavis.com/dna/gedmatch/default.aspx

Getting info from 23andMe:

https://blog.genomelink.io/posts/gedmatch-do-more-with-your-23andme-raw-data