I've only done it once but every time I do the sideflip I end up sliding down even if I let the stick on neutral... What do I need to do exactly?

getting 25 shines in 20 shines HYPERTHINK

You're talking about the wrong shine Ringo. To clip into cop secret slide on the wall with a coconut in your hand and throw it right before Mario lands. If you time the throw right you'll downwarp to the loading zone. Look at clips to see how far left you need to be also since you can't be over the green roof when you do it.

thank you @peaches_peaches_