How to use LCA in MEGAN to assign taxonomic info from custom db?

Hi all,
I am new to megan and want to use the LCA to classify my sequences against a custom database. This is how moving:
1, I searched GTDB genomes/MAGs by HMM and get all sequences of interests;
2, a diamond db were created and blastp were used to search my own sequence to this custom db.
3, now I have the blastp out from diamond, the fasta file of the custom db, a mapping file with database custome db sequence header (MAGs accession from GTDB) \t GTDB_tax.
4, based on these files, how could use LCA to classify my sequence against the mapping file and get the LCA based GTDB tax info.

I could import from outside the blast out-file. but not sure how to prepare and import the mapping file (synonyms file).

I check the community, see briefly about .abin file and mapping file, but still not figuring out where to start (specify the db location and import files)

thank you all.

#I use the community version of Megan, 6.19.6

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