To add one or more taxes in LSP.expert, you have to go to Settings > System values > Taxes.

Then, when creating a new client, you will find in the client creation popup all the taxes you added under Settings and you will simply have to select those that apply to this client in particular.

To add a tax to an existing client, edit the Overview panel of this client.

Note that you can define more than one tax for every client.

These taxes will then automatically be taken into account in your documents for this client.
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You can also edit the taxes directly in the document by clicking on your taxes at the bottom of the document. In the popup that opens, the system will ask you if you would like to update the taxes at the client level.

To apply a tax to a client, you have to define it at the client level (People > Clients). To add/edit a tax, click on Edit client.
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Note that you can define more than one tax for every client. You can also define negative taxes by simply adding a "-" (minus) before the number.
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These taxes will then automatically be taken into account in your invoices/quotes for this client.
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You can also edit the taxes directly in the document by clicking on Actions > Edit details.