Sometimes, you’ll get a match on AncestryDNA, 23andme, FTNDA or another commercial DNA test and not know who this person is, or how they fit in to your tree. Using DNA Painter you can eliminate the relationships that aren’t possible and narrow your list down to a few possibilities.
Firstly just enter in the cM shared or percentage between you and the person you’ve matched with, then it should automatically highlight the possible relationships.
The tool can be found here: https://dnapainter.com/tools/sharedcm/1000
You can also do this manually by visiting this post: http://familytreetips.co.uk/2017/01/24/dna-shared-between-two-people-checklist/