There are two methods, a) based on a list, and b) based on a template.
a) File-based joining
You can join up lemmas using a text file which automates the matching & joining process. The actual processing of the list takes place when you choose the menu option Match Lemmas () in WordList, Concord or KeyWords. Every entry in your lemma list will be checked to see whether it matches one of the entries in your word list. In the example, if, say, am, was, and were are found, they will be stored as lemmas of be. If go and went are found, then went will be joined to go.
b) Auto-joining based on a template
Or you can auto-join any of the entries in your current word list which meet your criteria: the menu option Auto-Join can be used to specify a string such as S or S;ED;ING and will then go through the whole word list, lemmatising all entries where one word only differs from the next by having S or ED or ING on the end of it. (Use ; to separate multiple suffixes.)
Prefix / Suffix / Infix
By default all strings typed in are assumed to be suffixes; to join prefixes put an asterisk (*) at the right end of the prefix. If you want to search for infixes (eg. bloody in absobloodylutely [languages like Swahili use infixes a lot]) put an asterisk at each end.
S;ED;ING will join books to book, booked to book and booking to book
*S;*ED;*ING will join books to book, booked to book and booking to book
UN*;ED;ING will join undo to do, booked to book and booking to book
*BLOODY* will join absobloodylutely to absolutely
The process can be left to run quickly and automatically, or you can have it confirm with you before joining each one. Automatic lemmatisation, like search-and-replace spell-checking, can produce oddities if just left to run!
To stop in the middle of auto-joining, press Escape.
With a previously saved list, try auto-joining without confirming the changes (or choose Yes to All during it). Then choose the Alphabetical (as opposed to Frequency) version of the list and sort on Lemmas (by pressing the Lemmas column heading). You will see all the joined entries at the top of the list. It may be easier to Unjoin (Ctrl+F4) any mistakes than to confirm each one... Finally, sort on the Word and save.
See also: Lemmatisation