[E6BAV8R]

1) When I plan the night before the flight and the weather changes the day of my flight , do I still fly with the same information I put on my Nav log?

The way I do it, and the way I teach it, is to have my students plan out all the airport information for the destinations as well as all the checkpoints and NAV information they can throw onto a Nav log. The day of the X-C is when they actually call FSS and get their WX and winds aloft and then compute all of their course headings, fuel, and time so it is accurate as possible.

I've never heard of CFI's telling their students to get the WX the day before the flight, I always do it on the day of the flight.

2) How do I find My True airspeed and RPM setting in the POH, I know how to interpolate but I mean taking the temperature from the metars and using the ISA which I don't understand.

Don't have a POH in front of me but ISA is ~2 degrees per 1,000 ft., if I am thinking of the right rate. Use that to get an estimate of the temp. at your cruise altitude and adjust accordingly. Also use the Winds and Temps Aloft forecast or grab the temp. from the FSS briefer.

3) When I use my flight computer to determine my Ground speed do I set the minature hole over 100 and than slide the mark I make with the pencil to my true airspeed?

Correct - the 'hole' will then read your GS after setting your pencil mark to the TAS line.