Ok so you should use any beard/facial hair cosmetic that scout has, if he even has one. Then, use any drink (Bonk, Crit-a-cola, or Mad Milk) and whatever scattergun. Also use the sandman because you have less health and it looks old.

Step 1: Hold the alt key on your keyboard
Step 2: Drag the mouse in a direction
You will then start moving slowly in that direction.

The voicelines you should use are as follows:
Move up!
Go go go!
Help!
Also do not capture points/intelligence/payload.

First, hold out the drink item.
Second, you need to walk like we mentioned earlier. Walk in random areas/directions.
Third, do not attack anyone unless they are a friendly, because you are old and don't understand what friendlies are. Attack them with your melee.
Fourth, spam the voicelines we talked about earlier.

Boom! you are now done with grandpa scout! Now go bully friendlies!