Should I erase pencil marks on primed trim before painting

If you must paint this stained wood, a light sanding with 4-O steel wool or 320-grit sandpaper should be fine. All you're doing is roughing up the surface coat, usually poly, which will let the paint "key" to the surface better.

Be aware of the finish originally used on the wood; only poly will take a latex topcoat well, while on most other finishes like varnish or oil-based finishes, latex will not bond to the surface properly; it may peel easily, or even bead up when applied. If this is the case, roughing up the topcoat is not enough; you will have to sand off the topcoat to get to raw wood, and on a profiled wood piece like mouldings, this can be difficult or impossible.

Also be aware of the type of wood; certain woods are "open-grain", and will absorb paint deeply into the structure of the wood, requiring many coats to "fill" the wood and get a solid color on top. Otherwise, the grain and any knots will show through the final coat until you put 3 or 4 thick coats on. If you have to go down to bare wood on an open-grained wood, you have to seal the wood with a product that is compatible with your paint base.

Getting a smooth finish without purchasing an expensive HVLP sprayer is fairly easy. Start with any good grade paint, and treat it with Flotrol acrylic additive at a rate of about 1/2 pint per gallon and mix well. If you get a real thick paint, you can increase the amount of Flowtrol up to one pint per gallon max. I use and recommend Purdy brushes. For trim, I like a sash brush, (angled style). Be sure to pick out a brush for latex paint, usually a combo of nylon and poly bristles. The ends of the bristles need to be very tapered and soft. Check out the Purdy ExtraGlides for latex.

Your technique is important also. Always try to finish with long smooth strokes only using the very tips of the bristles. Properly treated paint and the right brush used well will give you fantastic results. Good luck.