Here is my FoP handbook that Popanas saved from wizards old site:

I won't repeat the whole thing, but in general:

The best entry sort of depends on what level you are starting at. As Trandir mentioned, Kaorti is probably the easiest True outsider for a ground up build (2HD +2LA). Second to that is playing a Diabolus from the Dragon Compendium and adding on the Divine Minion template Diabolus is an outsider with the lawful subtype (+1 LA) which the divine minion template will switch to evil if you pick Sebek (+1 LA) for a total of a +2 LA. Both of these options makes you look a little monsterous.

If you are starting higher in level, ie you got cash to spend, then the ritual of alignment from Savage species is your friend. For 56,000k gp and 2,240 xp you can gain the evil subtype. Then you can apply this to any outsider, such as any +0 LA race with the otherworldly template applied to them or a planetouched raced. Otherworldly is a Faerun regional feat that was updated in Champions of Valor to be allowed to apply to any race. For can also pick up the outsider type for a +1 LA if you take one level in the savage progression break down for half-fiend, you can level the class at any time. These are better options as they let you play standard PC races with little to no LA, but you need the cash and the ability to build as if you spent it at a lower level than you got it.

There are some spell caster options for later entry into FoP which allow any race, but you will be waiting to level 10 or so.

Depending on which option you are stuck with, from there you just have to take 2 levels in one good Will save class (or outsider HD in the case of the Kaorti) and 1 level in another good will save class. Just select a combination that grants you access to both the Hide and Knowledge Arcane skills.

Outside of your Fiend of possession levels, you have to decide if you want to shoe horn spell casting abilites in or stick with augmenting your ridee'. There are builds posted in the handbook link. I am kind of partial to psionics. Ardents can make up a lot of ground due to how their manifesting ability works based on manifester level instead of typical progression. You can jump into thrall herd to pick up a slave to ride around on and manifest all day while in the sword. If avoiding squeezing in casting, marshal mixed with Fiend of Corruption levels can allow you more ability to augment your ridee'. You can also go the extra cheese route if your group allows it to mix in bloodlines from unearthed arcania. Those bloodline levels will add to your level based ability to enchant weapons from the FoP.

So take a look at the builds in the link and do your best to beg, steal, or barrow the 56k gp for the ritual of alignment to cut out build LA if you can manage it.