The only access to the tunnel is via J11A M20, the other access on the A20 is closed for road works at the moment so you need to get to J11 and join the M20 there.
If heading down you want to avoid the diversions on to the A20 then you could pick up the A2 and head for Canterbury.
Here you can leave the A2 and head into the centre and follow the signs for Hythe, once you pass the cricket ground and turn right at the lights you’re now on Stone Street, this road goes straight to J11 where you take the first left to join the M20. Bare in mind that getting across town can take some time during the day so if you don’t mind 10 mins of country roads there is another option……
Instead of coming of the A2 at Canterbury carry on until the next exit at Bridge. At end of slip road turn right. follow for 200 yds or so. At cross roads go straight over. Keep on this road for 5 mins or so, and at the end turn left, this is Stone Street, As above follow down the J11.
Hope this is of help.