This is my opinion on how you make a good MLP OC. Keep in mind this my not be that organized since I'm bad at organizing XP
And this is how I think it'll fit the canon (I'm really strict about following canon)
Anyway! Here we go!~

Races
Let's talk about pony races first. There are multiple to choose from. I'll list them all and anything you shouldn't do with them if I have anything I've seen that you shouldn't do with said race.

Earth ponies - I can't think of anything for earth pones
Unicorns - Don't make their horn super long like Celestia's. (This doesn't mean style wise.) There are some unicorns with a little longer horns but leave those really long ones to the alicorns. Also don't get them weather related talents. That's the pegasi's job. And make sure not to overpower their magic. Even unicorns with a talent in magic has limits.
Pegasus - Don't make it be able to perform a Sonic Rainboom. DONT GIVE IT ALICORN WINGS. Pony creator created ponies have this problem. Just give it normal pegasi wings. NOT alicorn wings.
Alicorns - DO NOT MAKE YOUR OC AN ALICORN. There's only 5 known existing alicorns. 
Zebra - As long as it looks like a zebra it should be fine.
Changeling - NO manes. They don't have pony manes. They don't have pony tails. They look all the same. For their disguise follow rules above. No cutie mark unless disguised.
Changeling (Reformed) No manes and make sure their tail looks like those in the show. And don't give it those antler thoughs. Only Thorax has them as far as we know. No cutie mark.
Bat pony - I'd stick to darker colors myself. But since we don't know much about them canonly.... Just don't make them an alicorn bat. Also try not to make it have the same color palette as a lot of other bat ponies.
Breezy - I can't really say much about breezies...
Deer - Or deer. Just don't give them manes or a cutie mark. Tyandaga's deer is an amazing example
Griffins - Not much to say about them either...
Last note about races: Don't make them hybrids. 

You got your race now? Good. Let's go with mane and tail styles next
My only note on this is don't copy off a canon character. Change it up a bit. I see way too many just copy RD's styles. Oh and don't give them the kirin tail unless they're a kirin.

Colors
NO RAINBOW - This has been way overdone. I don't think anyone can pull it off well.
Don't have a different color tail or mane tip. It's not canon at all. Just stick with the streaks. (Unless it's dyed)
No gradient
Try staying away from completely saturated colors
Avoid red and black unless you know for sure you can pull it off. There are a few good OC's that have red and black
Don't use the same color(s) you used for the body
Stay away from neon colors

Patterns are next
Stripes - Just avoid stripes at all costs on anything that isn't a zebra.
Spots - I don't recommend spots. 
Socks - Socks are amazing if they aren't gradient. 
Splotches - Splotches like Pipsqueak has are pretty cute in my opinion. But they can also end up looking really bad if you don't have a good color scheme.
CHOOSE ONE OF THESE. Not all. Besides splotches and socks. Those could work

Talent
Don't make them overpowered. Even the most powerful unicorns have weakness.
Don't make them amazing at everything. Ponies are usually only amazing at 1 or maybe even 2 things.
Try thinking of some unique talents that haven't been used or used very much.
Don't let them bring out the stars/sun/moon. That's an alicorns job and talent.

Cutie Marks
Try having one 1 or 2 things in the cutie mark. 
Make them simple looking. Like if it's a flower don't use much detail.
Have it represent their name or talent. This is usually the case with cutie marks.
Don't copy someone else's cutie mark design.
Avoid sun/stars/moon unless its astrology related

Names
Puns are a good way to go in my opinion.
Keep the names simple. Use simple synonyms.
Keep a pony's name 1 or 2 words.
Don't name them real human names

Backstory
Try avoiding cliches like dead parents and sob stories. (Say the girl who's most OC's parents are dead or missing) If yours does have dead parents think of a good reason why they died. Old age? Sickness? Things like that.
Don't makes them son/daughter of the canon characters
Don't make them best friends with canon characters ESPECIALLY the mane six
If you must have the mane six or canon characters in your story make it just small interactions

That's all I can think of for this... I'll leave with a few notes

1.) Don't make your character a mary sue
2.) Don't make a genderbent of a canon character and say it's an OC
3.) Don't add things to an existing characters or OC's cutie mark and call it your own