SMDs (surface mount device) are really awesome. Their low profile allows for a more seamless integration of lights on any project. However, depending on the size of the SMD you have, they can also be a real pain to work with.
Sewable often come attached to a PCB that allows you to sew the light directly to fabric. I've been able to find 4 packs of sewable SMDs for $4-7 depending where I look. How many sewable SMDs you need depends on the wearable you are making, so this can get expensive. You can buy SMDs (just the lights) in packs of 10, 20, 50, 100, etc. on eBay for less than the price of a 4 pack of sewable SMDs. You can make dozens of sewable SMDs for less than $15.
This Instructable shows how to cold solder conductive crimping beads (which you can get at any craft shop) to SMDs to create a sewable SMD for all your wonderful wearables. While using an actual soldering iron would probably produce better results, not everyone has one and those who do are not always skilled solderers---I fall into the latter category. I've lost many an SMD to the solder gods.
Step 1: BoM
Crimp beads (silver, brass, or other conductive material--do not buy beads that are "plated")
Various sized electronics tweezers, third-hand tool
Conductive thread & sewing needle
Coin cell battery & holder
Step 2: The Lights
SMDs can be very tiny, like grain of rice tiny, which can make them difficult to work with. I have a couple very fine brushes that I bought specifically for use with Bare Conductive Paint. The paint starts to dry fast and is conductive after about 15 minutes. The paint also acts as a glue. Basically, you can cold solder a conductive material (in our case, brass and/or silver crimping beads) to each side of the SMD to give you more space to work with and to allow you to sew the SMD to fabric.
If you have wall putty available, tack the bulb side of your SMD to your workspace--this will help keep the SMD from moving as you work.
Starting with one side, paint over the silver plates with a small amount of conductive paint. If you can get a small glob on, go for it, and then put your crimping bead down on the black paint. Give it 10-15 minutes and then add more paint around the sides of the metal bead and the SMD to help strengthen the connection. Repeat with other side and let dry.
I like to let my SMDs dry longer than necessary to decrease the likelihood of the crimping beads popping off while I'm working.
Step 3: Test
Once the paint is dry, test the SMD. Take a coin cell battery and place it between the bead crimps. If the SMD doesn't light up, turn the battery around and try again.
Once you've tested your SMDs, you can coat the black paint with a clear sealant like Mod Podge, clear nail polish, clear epoxy etc. This is to protect the paint and your connection. I suggest using a toothpick to apply epoxy if you go that route.
Step 4: Sew!
Now you are ready to sew!
To make a good connection between the SMDs and your battery, sew your conductive thread tight and close together. You don't want the loops too far apart. If you have it, use two different colors of conductive thread to make it easier to remember which side is positive and which side is negative.