Assuming it's for tracking the bass alone (not 'live' with others playing...), just place the mic at mid-height about 1 metre or so in front of cab. Room acoustics will play a part in the sound recorded, so moving stuff around and trying different locations (cab in a corner, or in the middle of the room etc..?) could be rewarding.
If there are others playing at the same time, it will be difficult to completely avoid 'bleed', but the mic at mid-cone height, horizontally halfway between cone centre and rim, 20 cm from the cab might be a place to start. More 'toppy' towards the cone centre, less towards the rim, distance to be adjusted by trial. Closer: less 'bleed', but less 'girth'.
Just my tuppence-worth, subject to completion, correction and/or contradiction from others. Hope this helps.