Depending on the knot or matted fur there are several ways you can go about handling it. First try to assess the severity of the problem area by find the base, gently brush the loose outer hairs out of the way and find where the fur is most knotted.
For small loose knots you may use your fingers to gently pull and try to untangle the fur enough to comb through it. This may be easier if you first work in some conditioner.
With tighter knots your best bet is to use a pinhead brush with plastic tips to work around the matted area just ‘picking’ the matt apart from the base. You only want to use the tip of the brush to catch, not going too deep or trying to pull the whole brush through. This is extremely well shown in the below video.
Failing that you may need to use a dematting comb or have to cut out the fur if it’s badly matted.
To do this find the base of the knot, while holding it between your fingers use your free hand to cut through the knot as close to the skin as you can with the tip of the scissors. Be sure to keep the scissors parallel to the skin or pointing slightly up and away, not towards your dog.
If you’re dealing with a particularly large matted area repeat the above cutting method in small sections, don’t try to take the whole thing in one section, follow this with a steel comb through. In extreme cases of badly matted fur you may have no choice but to shave out the section with a clipper, but this should only be done as a last resort.