To be very polite, peas should be crushed onto the fork - a fork with the prongs pointing down. The best way is to have load the fork with something to which they will stick, such as potato or a soft vegetable that squashes easily onto the fork. It's sometimes easier to put down your knife and then switch your fork to the other hand, so you can shovel the peas against something else on the plate, thus ensuring they end up on your fork.
The fork should not be used as a scoop but held so that the points of the tines face