1/2 cup baking soda onto the drain, add 1 cup white vinegar, wait for the fizzing to stop. Rinse with plenty of hot water.
Pour boiling water down the drain. Then add a half cup of baking soda and let it sit for a minute. Pour a cup of vinegar and a cup of hot water down the drain. Cover loosely and let it sit for 10-15 minutes. Pour another pot of boiling water down the drain to flush out the clog.
baking soda works great unless you have hair and debris in the drain. In that case I recommend using a drain snake like this. https://amzn.to/2N8e3Fv (affiliate)
Adding to the above suggestions I recently had a stubborn clog. I did the baking soda and vinegar and hot water. My addition was dish soap squeezed about 2 table spoons and kosher salt.
The salt scrubs and the soap degreased and slid the stuff down.