[RPG] When can I grant advantage/impose disadvantage

Having both advantage and disadvantage on your roll from any number of sources always means you have neither. (Even if you have a dozen sources of advantage, and just one source of disadvantage.)

From the Basic Rules section on advantage:

If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa.

"Neither advantage nor disadvantage" is the normal ability check.

A normal/typical ability check is rolled with 1d20 (modified for Proficiency and Ability scores). It takes "something extra being involved" for an ability check to accrue advantage or disadvantage. (Spell effects, class features, conditions, etc).

The D20
Every character and monster in the game has capabilities defined by six ability scores. {snip} These ability scores, and the ability modifiers derived from them, are the basis for almost every d20 roll that a player makes on a character’s or monster’s behalf.{snip} Ability checks, attack rolls, and saving throws are the three main kinds of d20 rolls, forming the core of the rules of the game. All three follow these simple steps.

1. Roll the die and add a modifier.

2. Apply circumstantial bonuses and penalties.

3. Compare the total to a target number.

Advantage and Disadvantage
Sometimes an ability check ... is modified by special situations called advantage and disadvantage. Advantage reflects the positive circumstances surrounding a d20 roll, while disadvantage reflects the opposite. When you have either advantage or disadvantage, you roll a second d20 when you make the roll. (Basic Rules, p. 4 / PHB p. 7)

• An ability check with advantage is rolled with 2d20, take the highest one.

  Example: Ranger. (Class feature Favored Enemy)

  You have advantage on Wisdom (Survival) checks to track your favored enemies, as well as on Intelligence checks to recall information about them.

• An ability check with disadvantage is rolled with 2d20, take the lowest one.

  There are a variety of things which will cause disadvantage on an ability check, such as a level of exhaustion.

  Exhaustion (Appendix A, Conditions)
  Level...............Effect
  1...................Disadvantage on ability checks

Those are the three cases of ability checks: normal, with advantage, with disadvantage.

Trying to create advantage

Whenever the party/players use the "help" action (working together) an ability check can be made with advantage.

Working Together (Sub category of Using Ability Scores)

Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action. A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.

Where it makes sense

You as DM can, whenever you feel that the circumstances warrant it, apply either advantage or disadvantage based on what the character is trying to do, and what judge makes it easier or harder to achieve based on their situation and their description of the attempt. (from Advantage and Disadvantage)

The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.