Installation Steps:

1.: make sure the following modules are installed:calendar
date_popup, date_timezone, content,content_copy ,nodereference,
number, userreference ,date, taxonomy, cck_referential_integrity, resource_conflict(version >= 6.x-2.x-dev !!)  and for commercial use the ubercart core modules.

2.: install the agreservations modules in the following order (assuming you want online booking and payment functionality):
    1: agreservations,
    2: agres_bookings
    3: agres_categories

    if you want timebased restrictions and seasonally different productprices:

    4: Agreservations Timeframe
    5: Agreservations Restriction
    6: Agreservations Rate

(agres_onlineformj is obsolete by now.)

3.: under http://yoursitesurl/admin/content/node-type/agreservation  enable the agreservation contenttype for Resouce Conflict checking by choosing the Resource Conflict datetime field and the booked resource reference field to check for conflicts, obviously these are: Reservationdate and unitreference. it should look like this:
 

4. add at least one node of type agres_category like: hotel room, car, table,...

5.: add at least one (or more) agreservations_unittype like: standard, large,...if ubercart is enabled: in the ubercartshopconfiguration at  http://yoursitesurl/admin/store/products/classes make this contenttype a productclass. Mark the agreservation_unittypes as non shippable.

6.: add some agreservations units like: Room #1, Guestroom, Room#2, the boat,...

7.: the module contains a default view bookingcalendar which should be installed automatically. When you followed the above steps you should see a matrix of resources and units. You can click on the little '+' signs for each day/room and make a booking, you see the agreservation form prefilled with the correct day and room you clicked on. You can choose more rooms of course.

8.: the agres_onlineform module brings a searchwidget Block which you can activate in Administer->blocks. This would be for the frontpage for users to make onlinereservations.

9.: If you have ubercart installed and agres_bookings activated you will see a new order_pane in the Admin create order form. Here you can quickly check if rooms are bookable in the desired time and make the reservation. This is usefull i.e. if a guest calls you by phone to make a reservation.

10.: For Ubercart you can use your own templates for invoices. Agreservations provides an adapted standard template with the name customer_agres.itpl. You can activate it by going to Store administration/Configuration/Order settings/Customer settings and choose it. You can also make a copy of it and make your own changes and then choose your version. The correct directory for the templates is: sites/all/modules/ubercart/uc_order/templates.

11.: Activate / deactivate the appropriate cart and checkout panes.