I'm remodeling my bathroom myself and when I ripped up the old floor and marble threshold seen the 2 planks are in very rough shape.

I would like to replace these myself and know there's different ways of doing it, what is the recommended way? Thanks

http://i.imgur.com/PGOg3egh.jpg

Take the door off to start. Make sure that the sub-floor is dry. Even a cheap moisture meter like you can get at Harbor Freight will tell you if the floor is as dry as other parts of the floor. Cut tight to the tub. Put a spline in if you are working away from a groove, and nail through it. I would use latex sub-floor adhesive near the tub, so only add moisture barrier up to 2 inches from tub.
Undercut the casing or jamb to get a tight fit, if you need to.
If you have a choice of grain pattern on the replacement wood, use quarter sawn or vertical grain because it will not swell as much if it gets damp again.

Cut what tight to the tub? I'm talking about my finished hardwood flooring. Common #2 oak. The 2 pieces in the threshold need replacing. I can get a better picture. 1/2'' plywood underlay is going over those subfloor planks.