Tutorial Resize Bedframes+Declutter+Addons+Seperate Items

Hey Guys,

am pretty new to customizing.

I want to be able to

• resize e.g. bedframes (to a Toddler version)
• separate/split some objects
• declutter certain object (think this is simply just deleting or cut it out, right?)
• create some add-ons (e.g. existing loveseat = add-on sofa, single bed = double bed)

Probably it's simple and I just missed it or looked up for the wrong keywords. I mean a bunch of Simmers did that but how in the world does this work? Somehow I cannot find matching tutorials for these topics?  I checked also the online manual here but I couldn't find what I wanted to do...  At least not by those words and I just hope am not stupid for finding it    It's really frustrating me! Tried Google and Tumblr but nothing pops up for what I'm looking for.

It would be dope if you are a creator and could tell me or if you know where I can find matching tutorials.
Which tutorials do I need in order to do such things?

Thank you so much in advance, Simmers for your help and understanding