What size tank. you say converting so is your freshwater tank still up and running?
Its easiest to just shut down the freshwater tank. Drain it and take out all the old substrate.
For your first batch of water you could mix it in your tank. Always add salt to water and never the other way around.
Dont put the sand in till the newly mixed saltwater has had a chance to mix for 1 day. Add the canister filter (if tats all you have for water movment) right away along with your heater and set the heater to 80F. Mix the salt slowly once your tank water has gotten up to temp.
When adding the sand put the bag in the water and cut a slit and pour it out under water this way. YOU will get less of a storm cloud.
Skimmer can go in at any time but dont expect it to skim anything until after you have some fish in there.
What do you plan on putting in the tank. What do you want in the way of fish.
CHeck out our article on how to cycle your tank with out fish >>HERE