The way to introduce alien DNA into host cell includes
a. disarmed pathogens
b. biolistics or gene gun
c. micro-injection

To introduce alien DNA into host cells following methods are involved.

1. Micro-injection - recombinant DNA is directly injected into the nucleus of an animal cell. 
2. Biolistics or gene gun - In this method, suitable for plants, cells are bombarded with high velocity micro-particles of gold or tungsten coated with DNA. 
3. Disarmed pathogen -  in this method ‘disarmed pathogen’ vectors, which when allowed to infect the cell, transfer the recombinant DNA into the host.