It is simple to do, assuming the police are using genotyping instead of a full genome sequence for the match. (This is the common case today.)
Genotyping works as follows: there are thousands of regions of “junk” dna in the human genome that don’t code for anything. There is no evolutionary pressure to preserve the exact length of these regions. (They probably help the chromosomes fold up in ways that encourage/suppress the activity of certain genes, help the molecule be more robust, etc).
Since there is no evolutionary pressure to preserve the regions, they tend to be full of hard to copy junk like ATAT..... that tends to change every few generations.
Genotyping works by isolating a bunch of these regions, and using PCR to create millions of copies of each region (with ~one sample per region).
Finally, you put a drop of the each sample and some die on a electrophoresis gel, and apply some current. Different sized molecules move at different rates, so this gives you a crude measurement of the length of each molecule.
To fake a match, all you need to do is produce something that will trick the first step of all this into grabbing molecules of your choice of length, which will fool the rest of the process. If you want to see how that would work, read up on “pcr primers”.
Source: I used to work in a genotyping/sequencing lab as a bioinformatician, and worked on algorithms that did exactly this sort of analysis (not the forgery bit though).