Eagle:
Best practices creating designs with EAGLE (GR8BIT)
KTOWN’s Ultimate Creating Parts in Eagle Tutorial (Adafruit)
How-to: Prepare your Eagle designs for manufacture (Hackaday)
Better PCBs in Eagle

PCB Layout tips and Tricks

Ordering boards:
OSH Park

Photoresist:
PCB photosensitive film material/DRF Film photoresist
Thickness: 60um
Exposure energy: ?SST=7/21?=13mj/cm2
Resolution: ?SST=7/21?=25um
Adhesion: ?SST=7/21?=30um
Stripping time: ?2%NaOH,50 °?=35MS

Toner transfer:
Using parchment paper for toner transfer

Solder Mask:
Dry Film Solder Mask

SMD
SMD sizes

Design guidelines
PCB spacing / clearance vs. voltage
For uncoated external traces, 24 mils for 150V and 50 mils for 250V.