My process, turn the outside shape and leave about 1/4 in for small bowls or 3/8 for larger. Size the tenon for your chuck to grip, put about a 10 deg dovetail taper on the tenon and make sure the face the chuck jaws will contact is very flat. Also using the tail stock with a center, contact the bottom to make a center spot. I usually cut the dovetail shape with a 1/2 skew laid flat like a scraper. Reverse and grip with the chuck, turn inside and true up outside and sand everything you can get to. Reverse ona jam chuck using the tail stock center on the previously marked spot. remove the tenon down to about 1/2 - 3/8 in. Slightly concave the bottom up to the remaining part on the tenon. Remove from lathe, cut off the remaining piece with a chisel and sand bottom. Ready to finish.
I usually start by turning between centers using a spur drive, on rough log section bla nks I can make minor adjustments where the tail stock is located to give the best shape. This especially useful when turning natural edged bowls. For larger bowls I drill a shallow hole the same diameter as the spur and place the spur in it, this helps hold the spur in place and is a safety factor as the spur can't slip out. I turn bowls and platters up to about 20 in rough blanks, as I have a 3520 B.
I have an assortment of jam chucks for bowls, platters and vases. I make them by using 2 in thick pecan, or other hardwood cut into about 5 in circles that are drilled and tapped to fit the headstock threads. Screw it on the head stock and true up both sides as appropriate. I the glue and stack circles of 3/4 MDF up to the height I wans and use the tail stock to clamp and let dry. When dry true up. I also hollow mine so I can use them on vases that have a small neck or taper in at the top. This cheap and easy and you can make a variety of sizes and shapes. You can even part off the old stack and put a new stack when they are used up or no longer useful. I reshape the end if necessary to fit my current need.
I use an air powered drill to power sand, with 2 in disks, it will clean up the bottom real quick. On larger bowls power sanding on the lathe is much faster than hand sanding on the lathe.