This is what worked for me in Shield:
First, choose the Pokemon that you want to raise friendship for. in my case, it was snom.
add it to your party and go about your buisness. walking with it slowly raises happiness. If you have a soothe bell, give it to the Pokemon; it can be found in Hammerlocke in a house next to the stadium.
Do stuff in the wild area often: lots of cycling.
Do other things that encourage exercise, like the rotom rally or egg-hatching
Make sure to cook curry often and play with the Pokemon at your campsite
If you think it's at max friendship, show it to the boy in the house where you got the soothe bell. If it's at max friendship, he'll give it a ribbon. once you've determined that it is at max happiness, just use some candy to level it up, and it should evolve.
The big one here is CURRY. if your curries are good, your Pokemon will gain happiness very quickly.