Transmission!!???!!! HELP!!!

Retired truck driver, double clutching is simply waiting, that time you take to double clutch is waiting for the rpm to slow down.
If 1st gear is unsyncronized, when sitting at a light, put it into one of the synchronized gears to stop everything, then shift into low.
If you are going to try down shifting into low from 2nd or 3rd, you go to neutral, clutch out, engine rpm up, clutch in shift to 1st.
It takes experience to do this correctly, it's all in your head, sound,speed, rpm.
But do what RRR said, make sure that your clutch is working properly.