When to use this system

In core 5e there are two types of checks, individual and group. This is not meant to replace these for all cases. You should consider this system when the PCs reach a climactic moment, when they are working on a complex task, or when they are trying to accomplish something over time.

Individual checks: Use these for targeted checks with a specific goal, checks with a concrete single action, or checks where a pc can only attempt it once. Examples would be jumping over a gap, trying to get evaluate an NPC using insight, using the medicine skill to heal, or recalling a specific piece of information.

Group Checks: These are best used when the group is acting as a whole over a non-instant period of time, but where the outcome does not have a lot of depth. Examples include survival checks to get to a destination without getting lost, trying to sneak up on a camp, or searching a room. The result of this check isn't likely to be that different then using this system but there's no chance to expand. This is absolutely fine for cases where you just need a to inject some feeling of challenge to the world without making every minor action into a huge event.

Skill tests: This is the system presented below. In order to make this system worthwhile over a group check the DM and players must invest time and effort into creating the event. This is analogous to combat in that there chances for players to have agency in the process and make strategic decisions.

The primary things to remember when using this system.

• PCs suggest an action they'd like to take, the DM suggests a skill or two they can use, and after the roll the DM describes the result. PCs should almost never say "I make an (insert ability here) check" unless that's very appropriate.
• As best as possible, consider using a variety of skills. For example, in a chase you could suggest intimidation if the PCs ask which way someone went or survival to guess which path the enemy took through the caves.
• The DCs might initially seem low. There's a LOT of math that went into these DCs and they are chosen carefully. A low DC on one check is easy to overcome but when you make multiple checks the difficulty increases quickly.
• Follow the order of steps in creating the skill, the order of choices is central to maintaining balance in the system.
• Frequently reward the PCs for thinking outside the box. For example if the PC chooses to use an odd skill but can back it up with a solid connection to the character or creates a backstory element, give them advantage. If the PCs have a particularly awesome idea give them a free check.
• Use disadvantage rarely. This system is designed to not feel like a guaranteed win, and in the long run you'll do much better encouraging good behavior than punishing low effort.