A common method to enter attributes one-by-one when using your field knowledge is:
Open the map and attribute table side by side (or with a bit of
overlap, but not too much) .
In the attribute table, select "show selected features" at the left bottom corner of the window.
Make the layer/table editable.
Select the first feature you want to label in the map. You will see it in the table, so it's easy to edit it.
Then you select the next feature, edit, next feature, edit... save and finish.
If your label is in sequential order, you will be able to order by that field to see the features in order of priority. E.g.
PBH0001 (for planned borehole 0001)
If you name them -BH1, BH2...BH9, BH10, BH11- you will have problems at the moment of sorting. It's preferable to have plenty of zeros - many projects outgrow the amount of zeroes just because the person doing the design never thought they would be needed.
Note: QGIS, like most other QGIS applications, does not store the records in your preferred order. This might be annoying if you are used to MS Excel, but you will get used to it.