Making Sigils
Ok. To start.
There is no right or wrong way to make a sigil, in the sense that there's multiple ways to go about it- you're going to have to experiment with the method that works with you the best.
Sigil Methods
These first few types of sigils work best if you want to obfuscate the work, where the actual intent of it isn't meant to be known (at least not to the average person).
1. The first way people usually teach is to take your phrase, cut the vowels and repeating letters out of it, and then use the shapes in the remaining letters for your symbol. Like this. There's nothing wrong with this, and it's a pretty straightforward method.
2. Another common method is to make a wheel of the alphabet, and then trace lines connecting letters (using the same method earlier of only keeping the "important" ones in your desired phrase) until you have a shape. This is explained here.
3. You could also make an abstract design based on the feeling you get when you think of the intent. For example, for the phrase "I am safe", you may associate round shapes (like stereotypical clouds) with safety, as opposed to sharp ones (such as triangles/zig-zags).
The most common type of sigil that I personally use- such as the seals I put on ritual oil bottles to prevent the magic from leaking out- are simple pictographs depicting the outcome that I want. These are best used for utility or private workings, where people are unlikely to mess with it.
So... What do you do with it afterward? How do you use it?
This depends on the intent of the sigil.
People's opinions on whether or not you should destroy the sigil tend to differ, and the reason for this is that the different methods have different effects depending on how it was designed to work. (e.g. They think destroying the sigil is what sets the magic loose into the world to work, therefore the sigil is made to be destroyed from the get-go.)
If it's something you want people to see, such as an image that gains power when people look at it, then post/draw/whatever it wherever you feel like.
If your magic comes from secrecy, then you're not going to want someone to see it. If you don't want to destroy it, and you want to keep "feeding" it- keep the sigil in a secret place, such as inside an altar box. In this case, you would charge the sigil by anointing it, burning a candle on/near it, or otherwise making offerings and directing energy to it (or asking spirits to bless it).
Otherwise, the sigil is destroyed. The method of destroying it often correlates with the element you want to invoke. Draw the sigil on a piece of paper, and then do one the following. If you intend to dispose of the sigil in the wild, I'd recommend using unbleached paper.
You may also combine methods. I often anoint the sigil with some spell oil before ripping or burning it.
Now what? How do I tell if it's working?
Use whatever standard divination method you use for other spells. I wouldn't think about it too hard. If you're absolutely sure it's not working, or needs an extra boost, then either "activate" it again, or make a brand new one. I, personally, have accidentally made "dud" sigils before that had to have the designs completely redone since the first one didn't do enough.
Remember: creating sigils is a craft, and like all crafts, you should practice it if you want to get good.