I want to remove my muffler and just put a straight pipe extension from there out the back of the car, how would I do this?

 

Just weld it.
Thats what my buddy did on his Civic.
I believe he used a tig welder.
But you also will need to add the hangers.

But mostly it will depend on what size piping you are gonna use and how thick it is.

 

what size should I use? id rather not weld, I know how just dont own one or can easily access one.

 

Unbolt muffler secton. Get flange and some pipe, weld the two, bolt it back up. Hello...take it to a muffler shop.

Also...you dont need to make multiple threads about this.

 

Agreed with BlkCurrantKord.
Muffler shop would be your best bet right now.
Costs money. But it'll be done right.