My bullet-point summary is:

1. Thomson studied cathode rays and decided their properties were best explained by them being particles.

2. The cathode rays were deflected towards a positively charged plate / away from negatively charged plate – hence they themselves must have negative charge.

3. The size of the deflection meant that their mass must be incredibly small, much less than the lightest known particle which was a hydrogen atom at that time.

4. Particles with the same properties could be obtained from lots of different elements – suggesting they were present in all materials.

5. Thomson proposed the plum pudding model of the atom where electrons were dispersed throughout some kind of positive atomic matter.