To get started the first thing you need is the claiming tool. By default the tool to create claims is a golden hoe. Holding the tool will display your current claim blocks and the amount you used.

To create a claim now right click on one block to select a corner and on another block to set the other corner. If it was successful you will get a message that a claim was created, else it will tell you why the creation failed. Alternatively you can also use the command /flan addClaim to do that if by some reason you prefer that method.

Resizing claim is done by right clicking on one corner of a claim and then on another block to set the new corner. To check if a block is claimed you can use the inspect tool. By default this is a stick.

Subclaim

Claims can support subclaims. As the name suggest a subclaim is a claim inside a claim. Subclaims have their own permission and group sets that override the parent claim they are in (no you cannot create a subclaim inside a subclaim).

Subclaim permissions have an additional value "default" instead of just true and false which simply redirects that permission back to the parent claim. To create a subclaim switch to subclaim mode via /flan switchMode SUBCLAIM.

Then use your claiming tool the same way you create a normal claim. Note that the positions need to be inside the claim.

Opening the subclaim menu and editing subclaim permissions requires you to be in subclaim mode too.