How can I guarantee that the skeleton forms of MissingNo appear?

As explained on Bulbapedia it seems the only way to "control" what form MissingNo will have in terms of the Ghost and Fossil forms is to control your name or using the Time Capsule Glitch

Kabutops Fossil form

• This form appears through the old man glitch if w is the character in the third, fifth, or seventh slot of the player's chosen name.
• This form can be obtained through the Time Capsule exploit if the traded Pokémon is Pupitar.

Aerodactyl Fossil form

• This form appears through the old man glitch if x is the character in the third, fifth, or seventh slot of the player's chosen name.
• This form can be obtained through the Time Capsule exploit if the traded Pokémon is Tyranitar.

Ghost form

• This form appears through the old man glitch if y is the character in the third, fifth, or seventh slot of the player's chosen name.
• This form can be obtained through the Time Capsule exploit if the traded Pokémon is Lugia.

All 3 mention using the Mew Glitch however I suspect the Special stat requirement will be random

The Ghost form also has an extra way to do it

• this form appears through evolving .PKMNRB or Z ゥY at level 224.

You can actually control it exactly using the Mew glitch as well; the special stats you need are listed on Bulbapedia, to wit,

MissingNo. (Kabutops Fossil form) 182
MissingNo. (Aerodactyl Fossil form) 183
MissingNo. (Ghost form) 184

The easiest way to get the specials you want is to use the Ditto variant of the Mew glitch, as outlined on Bulbapedia: https://bulbapedia.bulbagarden.net/wiki/Mew_glitch#Ditto_glitch . If you're planning on getting Missingno. in Yellow, then these three forms are the safest to use to not corrupt the rest of your game too much.