The first step to being to create a sigil is to understand what you are creating.
The Google definition of a sigil is " an inscribed or painted symbol considered to have magical power." This is actually a decent definition, although it is somewhat lacking. A more apt definition would describe sigils as a drawing created with magical intent or a symbol that has been created to represent an intent. In a way, they are written or drawn spells.
Creating The Sigil
There are actually a bunch of different ways to create sigils. Even so, there is a more 'traditional' way to create them. And by traditional, I really mean it's tried, true, and most common.
What you'll need for this process is
a writing utensil,
a piece (or a few) of paper,
and a lighter/match/fire.
Start by writing out your intention as clearly and concisely as possible. Stay in the present tense and don't use negatives. "I am" statements are the easiest. For example, "I am happy." is a good one. "I will not be sad." is a bad one because it uses future tense and negatives.
Future tense is considered bad practice in sigil making because it is ambigous. "I will be happy" could mean "I will be happy in two minutes" or it could mean "I will be happy in one-hundred years." Negatives are considered bad practice by some because of the belief that the subconsious skips over negatives. "I am not sad" then turns into "I am sad." The last thing to avoid in your intention statement is "I want." This is because it skews your intention into wanting something, rather than getting something. "I want to be happy" turns into always wanting happiness, but not achieving it.
The next step is crossing out all of the vowels and repeated letters. You are trying to turn your intent into a symbol. Striking out the vowels make your intention look less like words and more like symbols. You won't be using repeated letters, so strike them out as well to make things look less messy.
"I am happy." becomes "MHP" (Whether you consider 'y' a vowel is up to you. I usually do.)
Now you have a line of letters that don't look like much. Making a symbol from these is fairly easy. Put all of the letters together however you'd like. It will probably look very messy at this stage, but that's okay. Just make sure that all the letters are connected. Now, keep redrawing your "symbol," adding and retracting things however you see fit. Add things, take things away, change things, over and over again. Get lost in it, you should find it becomes kinda meditative. Do this until you are left with a symbol you are pleased with. Now you have a sigil! Yay!
Now it's time for charging and casting. You have your sigil. (I would suggest redrawing it on another piece of paper and ripping or cutting away any excess paper) To charge it, you fill it with the intent it was made for. Meditating on it is a popular one, but be creative, there are other ways to go about it. Once you've charged it, it's time to cast. The most common way to do this is to burn the sigil. There are other ways to go about it though, such as burying it, ripping it up, letting it disolve in water, meditating intensly on it, etc.
Other Methods for Sigils
Sigil practice isn't one that is set in stone. One of the most common variations used in creating sigils is the type of alphabet used. I personally created my own alphapet for sigil crafting. Popular ones are theban, runic, and greek alphabets, but some people even use fictional alphabets such as circular gallifreyan.
Some people use letter grids in order to create their sigils. This makes the drawing process easier, and also gives the sigils a very unique look. I have found that it doesn't work as well with more letters. You have to keep the intent statement pretty short in order for them to not look messy. Some people don't even use letters for sigils. Instead, they simply draw something that symbolizes their intent without going through the same process with the letters.
It is important to make sigils in a way that speaks to you. For instance, if you want to use the vowels in your sigil making because you feel like it will represent your intent better, then do it. If you want to make your own alphabet or connect to a fictional alphabet, use it. If the best way to cast your sigil is by drawing it on your window, then do it. If you want to charge your sigil by putting a toy on it that you think represents your intent, go for it.
Have fun sigil crafting!