Ok, so I'll assume that you used Mewtwo2000's tool to add a slot. Go to projectm->pf->menu2->sc_selcharacter.pac (this is for p:m 3.6, btw and you need brawlbox)
Open char_bust_tex_lz77 and you'll see a lot of MiscData[number] here. Each MiscData contains the CSPs of a character. This link (
http://smashboards.com/threads/guid...-m-3-6-csps-bsps-rsps-and-stock-icons.407758/) contains a list of which numbers correspond to which character.
Right click on the MiscData of the character you edited, and select Import->Texture and select the CSP you want to add. Next, you need to rename the texture as "MenSelchrFaceB.<number>"
Eg: if I added a slot to Samus, I would open sc_selcharacter.pac -> char_bust_tex_lz77 and right click on "MiscData[3] and then Import->texture and select the CSP. Next, right click on the picture you just added and select "rename". I'll have to rename it as MenSclchrFaceB.040.
Save and exit. The costume now has a CSP.
Now go to projectm->pf->info->portrite. You should see a lot of brres files. The link has a list for which file corresponds to which character. Copy and paste one of the files but make sure to rename the pasted file. Open this file and expand the "Textures" folder. Right click on the picture and select "replace". Choose the BP you want to add and apply.
Eg: For Samus, I'll copy InfFace039.brres and paste it in the same place. I'll rename the pasted file as InfFace040.brres. Open the newly renamed file with brawlbox, expand the "textures' folder and replace the picture.
Save and exit. The costume now has a BP.
For RSPs, projectm->pf->menu->common->char_bust_tex and again, you will see a lot of brres files.Refer to the link for which file corresponds to which character. Open the corresponding file with brawlbox and right click on MenSelchrFaceB<number>. Click Import-> Texture and select the picture you want as your RSP. Rename the newly imported RSP in the same way you renamed your CSP.
Eg: For Samus, I'll open MenSelchrFaceB030.brres and right click on MenSelchrFaceB030 and click on Import->texture. Select the RSP you want to use and click OK. I'll rename the newly imported RSP as MenSelchrFaceB.040.
Save and exit. The costume now has an RSP.
Finally, just copy and paste the FitCharacterXX.pac and .pcs into the corresponding folder in projectm->pf->fighter-> <charactername>
Eg: I'll go to projectm->pf->fighter->samus and rename the .pac amd .pcs files of my custom costume to FitSamus09.pac and FitSamus09.pcs and paste it into the folder.
That's it! This took longer to write than I thought. This may seem a little complicated but you will get the hang of it soon enough. Hope this helps.