1. Click "Create Server"
2. Change the name of the server if you want.

3.Select the map.

4. Wait until you are inside your own server.

5. Foward the ports on your internet.

Bravo! Now you can farm achievements!

Step 1 : Go to Properties and then press "Set Launch Options" and then type -console.

Step 2 : Host a server.
Step 2 Opinion : Let other can play the server so you can kill zombies and get achievement.

Step 3 : Enable sv_cheats 1

Step 4 : Try if you can do it by typing give fa_mac10 it should give you Mac-10 infront of you.

Congrats! Now you can spawn items!

NOTE : If you want to farm achievement I recommend you to set a public server that open to everyone so you can farm!

NOTE 2 : If you host a server and then set sv_cheats 1 and spawn weapons, you can't farm achievements.

"impulse 101" to spawn every weapon
NOTE : If you use impulse 101, it could be really messy and waste time getting things you wanted out of that piles of weapons.

"givecurrentammo" to fill your inventory with your current weapon's ammo
NOTE : It will fill you ammo to the brim, no exception.

"give <item_name>" to spawn an item. For example : give fa_1911

Here's the list of item names:
note : The item code of MP5A4 is actually fa_mp5a3. Don't get it wrong.



Handguns:
fa_1911 - Colt 1911
fa_glock17 - Glock 17
fa_mkiii - Strum Ruger MK III
fa_sw686 - S&W 686-6
fa_m92fs - Beretta M92FS
Rifles:
fa_1022 - Ruger 10/22
fa_1022_25mag - Ruger 10/22 ( 25 Bullets )
fa_cz858 - CZ858
fa_sako85 - Sako 85
fa_sako85_ironsights | Sako 85 ( No Ironsight )
fa_sks - Simonov SKS
fa_sks_nobayo - Simonov SKS (No Bayonet)
fa_jae700 - Remington JAE-700

Shotguns:
fa_500a - Mossberg 500A
fa_870 - Remington 870 Police Magnum
fa_superx3 - Winchester Super X3
fa_sv10 - SV-10
fa_winchester1892 - Winchester 1892

Machineguns:
fa_mac10 - Mac-10

Military:
fa_mp5a3 - MP5A4
fa_m16a4 - M16A4 (ACOG + Handle)
fa_fnfal - FN-FAL
fa_m16a4_carryhandle (Pure)

Archery:
bow_deerhunter - PSE Deer Hunter


Melees:
me_abrasivesaw - Abrasive Saw
me_axe_fire - Fire Axe
me_bat_metal - Baseball Bat
me_chainsaw - Chainsaw
me_cleaver - Cleaver
me_crowbar - Crowbar
me_etool - E-Tool
me_fubar - FUBAR
me_hatchet - Hatchet
me_kitknife - Kitchen Knife
me_machete - Machete
me_pickaxe - Pickaxe
me_pipe_lead - Pipe
me_shovel - Shovel
me_sledge - Sledgehammer
me_wrench - Wrench


Explosives:
exp_grenade - Grenade
exp_molotov - Molotov
exp_tnt - TNT

Items:
tool_barricade - Barricade Hammer
tool_extinguisher - Fire Extinguisher
tool_flare_gun - Flare Gun
tool_welder - Welder
item_bandages - Bandages
item_first_aid - First Aid
item_maglite - Maglite (Flashlight)
item_pills - Pills
item_walkietalkie - Walkie Talkie
item_zippo - Zippo Lighter
item_gene_therapy - Gene Therapy

Thanks to Kiro for the old source.

Thanks to this guy.

Thanks to everyone for checking out my guide.

Thanks to Alex for the spawnable item code.