How do you combine different strains

This question probably has been asked before. I tried searching for it but with no results.

How do you combine different strains? so you end up with (ex) one plant that is part white widow and AK-47.

I really would like to learn how to do this.

Thanks for your responses.

jjar7266

to be honest i really dont know to but i think it has something to do with like say i have a female(ill use your example) white widow, i would need a male ak47 to pollinate it? im not sure, thats just my theory. It makes sence to me.