You can either give it pp ups, rare candies, level it up, or give it the soothe bell but the rare candies work best.
There are various things that you can do to make your Pokemon more friendly. For example, you can feed them berries.
Battle with them A LOT or you can play with them in pokemon amie.
The more you train your poke'mon, the more friendly they will become with you.
If you have a magmar with you on your team that will help.
it takes ten steps for a pokemon to be friendly to you
It's usually very hard to have a Crobat that isn't already really friendly towards you in Pokemon LeafGreen. This is because to even have a Crobat you need to level up a Golbat that is really friendly towards you. To make a Pokemon more friendly you simply need to keep it in your team for a while. You can also take the Pokemon to Daisy Oak in Pallet Town to give it a massage.
Put a Pokemon with flame body/magma armor in party and 500 steps
Keep them in your party and battle after a while they grow friendlier
Making it level up, give it poke blocks, giving it zinc, protein.....etc, but if you make them faint, they'll become unfriendly. A faster way is to let them hold an item called soothe bell, it helps making the Pokemon more friendly
luxuray ball
The more you train your poke'mon, the more friendly they will become with you.
Luxury balls make it much easier to make a Pokemon friendly with you. Making a Pokemon friendly causes certain types to evolve and increases the power of the move return
Lots of training & musicals.
If you want to make your pokémon less friendly, you should battle and let it faint. Another way to make it less friendly quickly is to give it Revival Herb.
evolution stone
It is a special pokeball that has a catch rate between a pokeball's and a great ball's. If you catch a pokemon with a luxury ball, the pokemon will grow friendly to you much faster than normal. This is good because there are several pokemon that will only evolve if they like you a TON. Some of these are Munchlax, Togepi and Golbat
You can't make the eggs hatch faster, but you can play Pokemon Crystal Version on the Nintendo 64 with Pokemon Stadium 2 which has an option to increase the gamespeed.
give it a soothe bell
x speed