best way to reduce 4 k to 1080 after edit

I'm very new to FCP... I've seen some very high quality 1080 video on youtube created from an original 4k file. I've read that its best to start with the 4k or higher, make the edits and convert to 1080. I relate this to working with RAW files from DSLR camera and converting to .jpg for printing. I'm guessing the concept is similar.
Whats the best way to to do this? I think It starts with changing the timeline to 1080, but i don't exactly know where to start. If there a good video out there explaining this please share. thanks for any help

By default when you create a new project (aka sequence) in FCPX, it automatically sets the characteristics to the first clip added to the timeline. For a 4k clip this will set the timeline to 4k. After editing 4k you can export it in 1080p or 720p. For web sources using H264 1080p or even 720p can produce very good results and it minimizes export and upload time.

Optionally you can create a manual 1080p project (aka sequence) and drop the 4k content on that. You just override the default for File>New>Project and pick "Use Custom Settings". This uses 1080p intermediate render files which can improve editing performance. You will still benefit from the 4k resolution and can crop/zoom into the frame and the full 4k "zoomed" resolution will be used on the final 1080p export.

If you are editing much H264 4k it is often necessary to create proxy files. It is generally not necessary to create optimized media -- only proxy. FCPX makes this very easy -- proxy files can be generated either during import or afterward. Set the Viewer to Proxy for editing and remember to set it back to "Optimized/Original" before final export, else your export will be at proxy resolution.

FYI, we run in to this a LOT. A shooter hands us 4K footage, we bring it into our 1080 or 720 broadcast Timelines, and it is blurry. 4K is unforgiving with focus and motion blur. It can help with quality, it mostly helps going close ups, pans, zooms that original 1080 wouldn't allow. But half the time the focus wasn't precise enough and we end up using other shots that are sometimes 1080 or 720 originals. So don't think 4K is a magic bullet. Be very careful using it. It has potential, but you have to put the work in. Again, I've seen 720 shots that re of better quality than 4K shots, it isn't magic.

Just FYI.