Talk about going from one extreme to the other! Feathers are Very sharp. The selection pack would be a good idea, try the Gillette Yellow's may be a good place to start.
Echoing the above, going from a cartridge to a fixed head requires a very different technique. With the cartridge, you don't have to think about pressure or blade angle, and to an extent perfect lather. Now all these three factors need to be monitored, and any failings will result in burn / bleeds.
This are my personal thoughts:
- Lather
Spend some time practising making up a batch. Keep adding water drop by drop feeling how the consistency & lubrication changes with the more you add. You are looking to get the consistency of fine whipped egg white. Typically, the ideal point is just before the lather falls apart with too much water.
- Blades
They are all different - just as everyone's face is different. You may not hit on your ideal blade until you try your several. Additionally, blades perform differently in different razors.
- Pressure
Try your best not to apply any pressure on the razor, let it's weight do the work.
- Angle
Try to use a consistent angle. Use short gentle strokes & change the angle of the razor to match your face. You'll probably struggle with Chin & Neck.
- Reduction rather than removal
Your aim is to reduce the beard over several passes rather than remove it in one pass.
Never shave over an area with no lather - as it's the lather which is protecting your face!
Have a look at mantic59 on youtube. Probably the best instruction available. I really don't rate GeoFatboy - Blade buffing on first pass, Lather looks weak & a few other bad habits.
Good luck!