A lot goes into making a perfect sim. First you should pick the Gender, age, and voice settings. Another modifyable factor is chosing how the sim walks, an example is does he/she have a tough walk or snobish walk? You may also leave the walking setting as a default style, which is just an average walk.
Also located on this selector menue, there is the ability to adjust the family settings of the sim. This is where you are able to set relations between the Sims in this household (ie: Mother, Father, Husband, Wife, Brother, Sister, Child, or Room mate.) The next part of your sim's design is it's name. I would recomend naming it something suitable to your own standards, something you like.
The next step of designing your sim is adjusting its body and face fetures. This is where you decide wether your sim is fat, thin, pretty, ugly, etc. Personally I love the new settings in "The Sims 4" regarding these factors because they are much more realistic than in previous Sims games. The new software allows for the player to adjust the size of the sim's limbs and parts of the torso seperately, making the contrast of body parts much more relateable to people in real life. In order to access the face and body modifier, you need to click on either the face or body of your sim, whatever one you are trying to edit. Another interesting tip about the face settings is that if you double click on the sims face, it will allow you to adjust the features more thoroughly.
Once you have finished creating your sims physical appearance, you'll want to create a great style for your sim. This is when you can get creative with all of the fun and nice clothing the game has to offer.