There are times in small unit warfare that infiltration tactics are needed. 

Infiltration is used when

you want to execute movement undetected
you want to move into and through enemy territory
you want to occupy an area
you want to position your group into the enemy’s rear area
to give an attack advantage.

Infiltration forces are lightly equipped and stealthy.
This allows them to slip through the front line for strategic attacks
Infiltration can NOT defeat the defense by itself, so it must be used as a supplement to another maneuver.
Modern infiltration tactics are heavily influenced by 3 main sources:

French Infiltration Tactics
Laffargue Infiltration Tactics
Hutier Infiltration Tactics

French Infiltration Tactics
1915
The Theory

the first wave of infantry penetrate as far as possible
follow-up waves then attack strong points. 

This was partially adopted in battle, but German counterattacks were difficult to reinforce or hold against.
Laffargue Infiltration Tactics
Captain Laffargue

French captain
Injured in battle of 1915
Proposed a better idea

The Theory

A small infantry unit with mobile firepower to deal with machine guns.
Penetrate gaps and weak points with heavy fire
Strong points would be temporarily neutralized by fire or smoke.
Then new waves would encircle and attack each one.

Hunter Infiltration Tactics
German General Oskar von Hutier
In 1918 used Laffargue’s insights to supplement his tactics
The Theory
First, a brief and violent artillery attack of the enemy front lines (suppress the positions)
Then, the artillery would be shifted to the enemy’s rear to disrupt

lines of communication
artillery
logistics
and command/control nodes
meant to

provide confusion
weaken counterattacks
weaken concentrated fires
weaken the ability to fill gaps/penetrations.

Then light infantry would

Evade and bypass fortified positions
identify gaps in the front line
be followed by more heavily armed units.

Then heavy units would attack the strong points and yet other forces would enter gaps shortly after to further attack the strong points, which would quicken the collapse of the entire front.
In 1918, this was successful against the British and French

TPP 130: Tactical Maneuver: Infiltration Tactics from Ken Jensen
Reasons to use Infiltration Tactics

Direct attack from lightly defended positions
To flank or rear attack strongly defended positions
Disrupt enemy operations
Reconnaissance or surveillance

If you are wanting to succeed at this, you must be a force that:

Has trained together
Established command and control
Is small for stealth
Is large enough to get out of trouble

Infiltration Lanes

We will stick with a single lane of infiltration since, in a defense position, you probably will only have a small force.
Rally Points
Rally Point is an easily identifiable object where the force can assemble or reorganize if dispersed
Rally points should be defensible as well as away from normal routes of movement.
Initial Rally Point
Assemble for initial instructions, behind friendly lines
En Route Rally Point