How do scientists create specific mutations?
Thats basically the oldest method to induce such mutations. For this purpose either radiation (x-rays) or mutagenic chemicals (like Ethylnitrosourea) have been used for this purpose. This method is undirected, so you never know what the outcome will be untill you see the off-springs. Using this method to get specific mutations is relative difficult. Its used for the detection of genes which deliver visible phenotypes (for example it has widely been used for the analysis of pigmentation in mice, since you can easily spot mutations).

Newer methods can target genes directly and either introduce or remove sequence areas. These techniques make use of homologous recombinases. In short you bring in you gene of interest in the modified version on a plasmid into the cell which you want to change. The plasmid also expresses the recombinase (this can be under a cell specific promoter to target specific cells) which then recombines the DNA between the plasmid and the genomic DNA.

An example for that is the FLP-recombinase system (image from the Wikipedia article on it):



Other examples for site specific altertions are the Cre-LoxP-System or the Crispr/CAS-System.