Here is a pretty lame "Friendly" IDI theory (best I could come up with):
1) Perp enters the house at while Ramsey's are away on Christmas day, through the metal grate and basement window.
2) He is wearing gloves, has a brown bag with a rope, cord, a rag and a flash light in it. His motive is to sexually abuse JBR.
3) He hides in JAR's room, knowing it is empty and a good hiding place but also a good place to hear everything going on in the house.
4) Ramsey's arrive home, JBR and BR are put to bed and JR and PR go off to bed too. Perp patiently waits for all things to go quite.
5) Perp then goes to the next room which belongs to JBR. He leaves the brown bag and the rope in the room but brings the flashlight and the rag. Wakes her up using the flashlight to shine on her face. She is surprised to see him there and doesn't get scared as she knows him.
6) Perp says "Lets go eat pineapple!" They go to the kitchen, perp grabs bowl, spoon and pineapple and sets it out. No fingerprints on the bowl/spoon as he is wearing gloves. Either JBR eats the pineapple with her fingers or the perp spoon feeds her.
7) After she has had enough pineapple, perp asks if she wants to go down to basement for "playtime". Perp has already grabbed her favorite doll and blanket from her room so JBR doesn't get too afraid.
8) They go down to the basement and Perp starts to get pervy on JBR. Like stripping her, tying her and using a paint brush from the near by art supply box, to molest her. JBR starts to get really uncomfortable and before she gets into a full blown panic and screams, she is hit on the head with the flashlight and is knocked out.
9) Perp realizes she is still alive and makes the garrote using the cord he brought and chokes her, resulting in her death.
10) Perp is now overcome with guilt. He wants to leave the body with some dignity, so he wipes the blood with the rag, goes to JBRs room and grabs the first underwear he can find (doesn't care if they are size 12), returns and dresses her in them, wraps her in the blanket.
11) He now knows that if the body is found, he can come under suspicion as he is known by JBR and the rest of the Ramsey's and wont have an alibi.
12) So he heads up to the main floor, grabs a pen and a pad, and writes the ransom note, and leaves it on the steps (so anyone coming down the steps immediately finds the note).
13) Next he heads down again, tries to carry the body out of the tiny window and fails. Tries again by using a suitcase to get better elevation, but fails again.
14) Murder being on his conscience, he is not as steel nerved as he was when he entered the house. So he leaves the body and flees though the window and grate. Leaves doll in basement and brown bag in JAR's room.
15) Spider makes a web on the window corner after he is gone.
Feel free to blow holes in this theory. I don't know all the facts by heart so there are probably a 100 different mistakes in there. But if you can correct those mistakes by plugging in the correct facts, maybe this becomes a plausible theory.
Thanks for reading.