You cant Flush an A/C system all you can do is replace the system parts and recharge and vaccum test with the machine used to service A/C systems which is about 1600 bucks minimum. If you used a air hose and blow out the system it will be contaminated no matter what and will not work properly.
If you say the oil is dirty I suggest you just get it to a shop that is well know for good work and replace Condensor, Evaporator, Reciever drier and so on. Depending on the year (hopefully not R12 have fun buyin that) '93 or '94 til present is R134A. A/C system servicing is somethin a certified tech should be doing not the regular person. But choose what you want to do exactly first.