There are a few Utube videos on harvesting yeast..... It's a good place to start. Personally I top crop, but my fermenters are wide mouth. I sterilize a ladle in boiling water and drop it in StarSan to cool. I then lift the lid, and ladle off the yeast on top of the brew into a pint jar which is also sterilized. I typically do this 3 or 4 times during fermentation, and get a LOT of yeast. After chilling it in the fridge, it forms a nice yeast cake in the bottom of the jar, with wort on top. I normally just keep this wort..... or some of it. and pitch the works into a later brew. About 12 hours before brew time, I take the jar out, and warm it. Shaking vigorously to aerate whenever I happen to walk by it... Lid loosened enough to prevent pressure build up. This amounts to a very heavy yeast pitch, and I typically am percolating nicely within a couple of hours, fermentation mostly over in 3 days on a typical beer. You can pour the wort off, and replace it with a small amount of "generic wort" made from DME, which I did with my most recent brew because it had hops (Sorachi Ace) floating in it that I didn't want to transfer to my new brew. You could easily split the top crop and have more than enough to pitch several brews. Is is worth messing with the yeast cake when top cropping is so easy? If you ferment in a carboy, it's your only option.