Buy one of these
I have one, i am on my second 90 degree head. You have to lube these things a lot or at least the cheap ones. like every hour or so of use. Sometimes less.. I have had these things uncomfortable to hold hot. It is so nice to use though and spins real fast which then lets you use....
These bad boys. I do all my roughing with carbide or big cut stuff then switch out to these for finishing. The amount of control you have is second to none and its easy to get a nice finish if you use the right technique plus they go really small so you can get real crisp corners on stuff
Finding the proper D latching bits is annoying, I usually just grind the D and then superglue it in, it just takes a tap to remove it.
Also if you are porting an aluminum nicasil bore, these are 100 times less likely to chip at the port edge as the carbide bur's grab and try to pull chunks off. These are much less grabby in that aspect.
to get that pro finish I use one of these and before I am completely done porting start going in one direction back in forth, usually from a corner to a corner, then change it up and go 45 degrees but keep the same back and forth pattern really tight on the last pass.
the reason for this in my experience It is really hard to get rid of the divots the tool can make in one direction, however if you then change the angle the tool cant "fall" into that divot without riding on the high section beside it which takes a little off that part, then you change angle and repeat again till you get the desired result.
You are basically doing a crosshatch. It makes it way easier for the tool to not follow the same patch and dig in every time.
I keep a can of WD-40 for cutter lube.
Layout dye is cheap and makes laying out ports easy and visuals always help with getting good results. On the cheap if you have a sharpie and and air compressor you can put the tip at the nozzle and spray sharpie to scribe as well. it's messy though.
Pick up some small riffler and round files for chamfering or even porting in odd areas.
PAINTERS TAPE THE COMPRESSION AREA SO YOU DON'T HAVE AN OOPSIE. We have all had a bur jump out of a port or the spinning collet rub against the cylinder and leave a little mark.. this just makes it so that is way harder.. do the sealing surface at the top of the head too.
I am not a pro but this is things i have picked up.