Make sure to remove the old gasket.
Use a short piece of PVC pipe to seat the new gasket.
Use new gaskets for the Slip-ons.
Depending on how many times your slip-on have been removed you may need new clamps for them.
Make sure you have a swivel socket adapter to use. I installed the Fuel Moto 2-1-2 head pipe. There is a Fuel Moto video (on youtube) which illustrates the install.