Slip knots are typically better used for hanging. They stay tight, slide easily, and are easy to master as well. Try to wrap your pant legs as thin as possible because you don't want the diameter of the "rope" to be too thick. You could really hurt yourself or make your death more painful if you use too thick or too thin of a diameter.
I strongly suggest trying to get hands on rope. I've seen people here recommend 10mm diameter rope and I myself have 10mm. (rope diameter based on how thick your neck is kinda below the middle of your adams apple (or where it would be if you had one. Someone please correct me if I am wrong). You can find them somewhat easily for about 10-20 US smackaroos online, although the lengths vary (I recommend about 15-30ft depending on what you wanna do). If you use Amazon, you can ship the rope to package locker nearby and hide it after you pick it up.
If not there are suggestions down the page here under "Ligature Type".
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I'm glad you are strongly thinking about this