To start off there's three things that are most important above all:
Leadership:
There can only be one person calling strats. Even if they aren't working or going to plan, it needs to be followed to the letter. Once everyone starts jumping in with their own strat ideas it causes chaos and people lose track of what they're supposed to be doing.
Communication:
At the start of each round, ONLY the scrim leader talks (refer to the leadership section above). Now, while it's important to let people know what's going on where you are, don't go overboard. There's a fine difference between distracting people with banter and passing information. This also goes for when you're dead. Let people know where you died and how many opponents were there, then be quiet. If you're dead, you don't talk. It'll just stress out the people that are still playing.
Money:
Managing your money is paramount.
Example: First round of the match, everyone wants to buy a deagle. But if you lose the match, everyone is back to square one with no money. So have one or two people buy deagles and the rest don't buy anything.
Before the scrim starts, someone needs to be identified as the sniper. This person saves up money and only buys awps and such.
Overall, money management is up to the leader. You don't want to be the one who is asked to buy something specific and not have the money to do it because you decided to blow your money the round before.
Strats:
CT side:
This side is really simple, the positions are as follows:
1 mid with awp when available
2 A
2 B
The awper (when he doesn't have an awp) goes either A or B depending on the leaders decision at start of the round.
When 3 are A one covers cat, one long, and one moves between the two as support.
When 2 are A one covers cat and one covers long.
(They can also make a mutual decision to rush cat together or rush long together to mix it up against the opponent)
The two that go B can do the same by mixing up their strats, as long as B is held or prevented from being bombed.
T side:
Here's a few of the key strats as T
3-2 split:
3-2 split is simple, 3 CAT, 2 long.
The guys at long should buy long range weapons while the guys at cat buy short range weapons...Timing is important as to whether the cat and long guys converge at A at the same time or if long draws the CT's in and cat drops in from behind.
Next strat is semi confusing.
Around the World
Everyone needs at least flashes and nades
Everyone rushes b tunnels hard, one person throws a flash. Then everyone runs down to mid and one throws a nade at cat. Then you run all the way back through T spawn to the b tunnels and repeat the process until you decide to do one of these strats:
1) Rush B
2) 3-2 Split
3) Mid to A.
4) Mid to B
Keep in mind that while you're doing this, one or two people are occasionally checking behind the group.
Delay:
You just stay quiet at some spot outside of one of the bombsites. (near t spawn for b, in the double doors for a long a, and so on so forth.
That's all the basic strats and ideas that are key to scrimming.
Adios