How can a 15 year old boy make a dildo and not have to buy anything?

You can use any penis-like object with no sharp edges or points: people have used cucumbers, candles, even mop handles. Some guidelines:

Start with a bowel movement.

If it hurts, you're doing it wrong.

Thoroughly clean whatever you use before and after (or use a condom).

The anus itself is reasonably tough. The tissue inside the rectum isn't. NEVER put anything rough or sharp up there (including a fingernail). Don't use anything made of glass or brittle plastic.

Make sure you can hold onto whatever you're using. Believe it or not, it's possible for a dildo to slide all the way in, unless it has a wide base.

The anus does not produce its own lubricant, so you will need something like petroleum jelly, KY cream, etc.

Don't rush things -- start small: the idea is to learn to relax the sphincter muscle. Once again, if it hurts, you're doing something wrong.

Blood in the stool can indicate a serious injury. Be prepared to tell the doctor --honestly --what you've been doing. Don't worry. They've heard it all before.