Overcalls
When the Opponents have opened the bidding you can make and Overcall with a good 5+card suit, headed Q 10 or better.
This will disrupt the Opponents' bidding and convey to your Partner which suit to lead in case you end up as Defenders.

1. Overcall at the 1-level : 8+ points and a 5+card suit with SQ = 7 or better
   Usually 8 losers or less

2. Overcall at the 2-level : 11+ points and a 5+card suit with SQ = 8 or better
   Usually 7 losers or less

Suit Quality Test (SQ) of your long suit

SQ = number of cards in the suit + number of Honours in the suit
        (J and 10 are only counted as Honours when supported by a higher Honour)

Partner will respond with :

0-8   = Pass

9-15 = raise Partner's suit with 3+card support (9-12 = single raise   13- 15 = jump raise)

  bid a new good 5+card suit

  bid NT with stopper in the Enemy suit

16+ = raise to Game, or jump shift

Take-out Double
When the Opponents have opened the bidding you can make a Take-out Double (up to and including 4♦) to compete in the auction. You should have 7 losers or less and either one of the following two hands :

1. With 12-15 points : 0-2 cards in Opponent's suit, and an unbid 4-card major
                                   (or 43+ in the majors if neither major has been bid)
2. With 16+ points : any shape

Partner's response priorities are :

1. bid his 4-card major

2. bid NT with Enemy suit stopper

3. bid his long minor

1. 0-9 pts    = bid 4+card suit (but Pass with 0-6 pts if RH Opponent makes a bid)

2. 10-12 pts = jump bid 4+card suit

3. 13+ pts   = bid Game, or Cue bid the Enemy suit