This page presents conversion notes for using the Battlesystem miniature combat rules (in either of two flavors) with the current d20-style D&D 3rd Edition game rules.
Battlesystem I (1985 Boxed Set)
As per D&D 3rd Edition, you must start by recording each unit's total Attack Bonus, effective Damage Die (possibly converted, per Battlesystem section [8.5]), and total Armor Modifier (equal to AC-10).
Each unit's Movement Rate should be set according to the D&D Speed divided by 2.5 (in inches). Morale (and Discipline) is figured from a starting base of 10, adding all modifiers in section [4.2]. Morale Checks are made by rolling d20 + Morale and comparing against DC 20. (Alternative: This could be a 2d10 roll if you prefer the original system.)
When an attack is made, first make an attack check by rolling d20 + Attack Bonus - Armor Modifier. Also add in any normal D&D bonuses and penalties. (Note: You must add the negative of any listed Battlesystem modifiers, since in that system, positive numbers are better for the defender, and vice-versa. For example, figure Ratio Adjustments from Table 9 contribute -5 for a 5:1 figure ratio; -10 for a 2:1 ratio; and -15 for a 1:1 ratio.) (Alternative: Using a d20 attack check makes for a large random element in possible combat results; you may opt to replace it with a 2d10 or 3d6 roll, similar to the original rules.)
Finally, subtract this number from 30; it is this number that you then look for under the "Attack Roll" on the Battlesystem Combat Results Table, and cross-reference with the Damage Die to see how many Hit Dice of the target have been killed. (Alternative: You could instead re-number the left-hand column on your Combat Results Table, starting with "30+" at the top, working down to "-10/less" at the bottom, and thereby avoid this subtraction step during combat.)
All other rules, such as for Command, Initiative (using the d20-based "optional rule" from section [5.1]), Movement, and Magic remain unchanged from those in the Battlesystem Fantasy Combat Supplement.
Battlesystem II (1989 Perfect-Bound Book)
For this system, you must first compute each unit's Attack Die, found by adding their D&D Attack Bonus to a number of Conversion Points dictated by Maximum Damage per Round (see Battlesystem p. 104). For armor, simply record the unit's total Armor Modifier (equal to AC-10). Compute Hits as normal (on p. 105), and set Movement Allowance at the D&D Speed divided by 2.5.
The Morale Rating should be set starting at 10, and adding all modifiers found on p. 105. Morale Checks are made by rolling d20 + Morale and comparing against DC 20. (Alternative: This could be a 2d10 roll if you prefer the original system.)
When combat occurs, roll the given Attack Die for each figure and look up the "Hits per Attack Die Roll" exactly as normal (including any situational modifiers to the number of dice thrown). Then, for each hit, the target now makes an Armor Check by rolling d20 + Armor Modifier and comparing against DC 20; every success negates one hit. Any of the Battlesystem modifiers to Target Armor must be doubled, and then subtracted from this check. (Alternative: If you prefer the original d10-based system, the AR can be computed by calculating (30-AC)/2. Then, Battlesystem modifiers are used normally and the goal is to roll less than or equal to the target's AR on a d10 for each hit.)
All other rules remain exactly the same according to the Battlesystem Miniatures Rules.