How can I get my dog a colar on the sims 2?

If you've ever played around editting a community lot, you'll know that there are special items that can't be used in residental lots, like the clothing and magazine racks. There is a small display rack with dog/cat collars that comes with Pets. However, since it's an expansion, this display rack won't be in any of your pre-existing shops. That's easily fixed.

You could either add this rack to one of your pre-existing shops by entering the shop lot from the neighbourhood view, or, when you are in the neighbour view, if you click the Lots & Houses button, there should be a small petshop called Pepe's Pets which will have a pre-designed layout with all your furry critter's needs. Just place this shop down somewhere in your neighbourhood.

Take your Sim to the shop, buy a collar you like, and when you get home click on any mirror in your home. Choose "Change <pet name>'s Appearance" and your Sim will call the pet over. A small window will popup and under one of the tabs will be a list of collars you've bought. Select one and click okay and boom, your collar will be applied to your animal. :)

oh that's easy all you have to do is buy one then groom it and there will be an accessory button tap that then put it on!

I don't think you can, unless you have a cheat. Hope it helps!