Hmmm. First you must have the right intention and make sure you summon the right demon for the right purpose, there are many different demons out there and it's insulting to them to summon one up for no reason. Make sure you summon the right demon. Always remember that the laws of karma are still in affect.
Second, are you going to start small and work your way up or are you just going to summon whatever comes first to suit your purpose? I would recommend starting small. Like the other post said chaos magic has some great sigils to help you out but the hoodoo tradition has summons as well. You can look those up on your own.
Third, it's best to make a deal or contract when in a tranced state. I prefer to discuss matters with spirits in the state between waking and sleeping. This tend to take some focus because when a spirit (demon or otherwise) answers back it can be unexpected or scary! It also doesn't help if you're not good at astral projection and stuck in your body but can't move! I speak from personal experience.
Fourth, Always treat spirits with the utmost respect, unless you do a proper binding ritual the demon has power and it will use it! The last thing you need is to insult the entity, lots of people believe that they'll just be friends with the demon but that always doesn't work!
Fifth, when you use a black scrying mirror to see a demon they can appear scary, it's always best to ask the spirit to use a human form, also beware. When making a contract with a demon they can "technically" do what the contract says but do it differently. Example: I summon up a demon for protection, I end up getting attacked and I can't fight back and I'm not protected, the demon technically kept the contract because I didn't say HOW or WHEN and under WHAT Issues I wanted to be protected from (the demon could just as easily hurt you or hurt someone you care about to hurt you emotionally). Remember contracts are very important and all about wording.
I don't know why you want to summon a demon and i will try not to lecture you. I left some summoning rules out on purpose to encourage you to study, hope this doesn't make me look bad but I sincerely want you to be prepared for what you are planning. Please remember that karma still plays into the universe and what you do or have the demon do on your behalf will come back to you and also good luck. You never learn if you never take any risks but please do it responsibly and own the outcome if it works. Ciao!