Warning only use this info if you are capable of performing electrical work and or willing to put you or someone else at risk of death.

When we had an ice storm last Jan -Feb I bought and hooked up several generators for the 2 weeks we were out of electric, I  have a disconnect at my house but most don't . I went to the breaker box and turned off the main breaker, if your not competent enough to do this leave the gen to someone else . Yes if you forget to do it it will back feed the line , but with 20 amp it won't power much down the line, if you have plenty of neighbors it won't keep the breaker on the gen from throwing . If you have a PTO gen pulling 200 amps then you could do some real damage far from home, ( not that the small portable gen can't kill anyone ) but in an emergency you do what it takes to get by . 

 If you have made it this far and after you have your main turned off you are disconnected from the power company , run a cord to a 220 breaker you won't use and is big enough to handle your generators amps and wire it up , this will serve as your main breaker for the generator and home , make sure you have a plug for the 220v outlet on the generator and plug it in,  turn your new main off along with all other breakers  ( the one you wired your cord to ) and fire up your gen , let it warm up some and then turn your new main on and  begin turning on what you need in the home . With  20 amps  it won't take much to pull the gen down and trip it's breaker  you will then find out what you can pull.

 This will get you by till you can find a system that will meet your electric code and can have ready when needed. I would also check with your landlord  to see if he approves of you doing this .

 This will get you by in an emergency , but the best is to check your electric codes and see what is needed to meet them before you have to do a quickie job in an emergency .